Element for a preform, a fiber production method and an optical fiber drawn from the preform

ABSTRACT

A preform element, its production, and fiber production methods from preform assemblies. The preform element has a length and a center axis along its length, a first and second end defined by its length and an outer preform element surface. The preform element includes a plurality of longitudinal structures disposed to form longitudinal hole(s) in a background material. At least one slot extending from its outer preform element surface and intersecting at least one of the longitudinal holes, wherein the at least one slot does not fully intersect the preform element. The preform element may be a preform center element or a preform ring element and may be assembled to a form part of a preform assembly for an optical fiber.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. application Ser. No.15/774,780, filed on May 9, 2018, which is a U.S. national stage ofInternational Application No. PCT/DK2016/050364, filed on Nov. 10, 2016,which claims the benefit of Danish Application No. PA 2015 70724, filedon Nov. 10, 2015. The entire contents of each of U.S. application Ser.No. 15/774,780, International Application No. PCT/DK2016/050364, andDanish Application No. PA 2015 70724 are hereby incorporated herein byreference in their entirety.

TECHNICAL FIELD

The invention relates to a microstructured optical fiber comprising oneor more holes extending along at least a length section of the fiber andin particular to the production thereof. The invention comprises apreform element for the optical fiber and a method of producing thepreform element.

BACKGROUND ART

Microstructured optical fibers comprising one or more holes extendingalong at least a length section thereof are well known in the art andcomprise in particular photonic crystal fibers, in the followingreferred to as PCF. The microstructured optical fibers are fibers havinga core surrounded by a cladding region having a plurality of inclusions(sometimes called cladding features or microstructures) arranged in abackground material, typically in a regular array. The inclusion may begas, liquid, or solid inclusion. In principle the inclusions could bevoid, but in practice the voids will normally comprise some gasmolecules.

The microstructured fiber may for example be of silica glass. Othermaterials may be added to the silica glass in order to alter therefractive index thereof or to provide effects, such as amplification oflight, sensitivity, etc.

Microstructured optical fibers are usually produced by first producing apreform having a desired cross sectional structure and thereafterdrawing the fiber from the preform at a suitable temperature ensuringthat the material of the preform is sufficiently soft while not fullymelted. Such a method is for example described in U.S. Pat. No.6,954,574. The preform is produced by stacking a number of canes, eachhaving a longitudinal axis, a first end and a second end, at least someof the canes being capillaries each having a hole parallel to thelongitudinal axis of the cane and running from the first end of the caneto the second end of the cane. The canes are formed into a stack, wherethe canes being arranged with their longitudinal axes substantiallyparallel to each other and to the longitudinal axis of the stack providethe preform. Thereafter the preform is drawn into a fiber whilstmaintaining the hole of at least one capillary in communication with asource of fluid at a first pressure whilst maintaining the pressurearound the capillary at a second pressure that is different from thefirst pressure, thereby the hole at the first pressure becomes, duringthe drawing process, a size different from that which it would havebecome without the pressure difference.

U.S. Pat. No. 8,215,129 discloses a method of manufacturing amicrostructured fiber, comprises: providing a preform comprising aplurality of longitudinal holes; mating at least one, but not all, ofthe holes with a connector to connect the hole(s) to an externalpressure-controller; drawing the preform into the fiber whilstcontrolling the gas pressure in the hole(s) connected to thepressure-controller. Thereby during drawing of the fiber, the size ofindividual holes can be controlled by individually addressing each holein the preform and changing the pressure in that hole. If several holesare addressed simultaneously, then the structure which emerges in theactual fiber can be altered during the drawing process. The connectormay comprise a plurality of chambers, each chamber being connectable toan external pressure source and the connector and preform may bearranged so that at least a first one of the longitudinal holesterminate in a first one of the chambers and at least a second one ofthe longitudinal holes terminate in a second one of the chambers;wherein the preform is arranged to pass through one or more of thechambers such that at least one of the longitudinal holes terminate in achamber arranged next to a chamber through which the preform passes.

U.S. Pat. No. 7,793,521 discloses a method of fabricating a photoniccrystal or photonic band gap optical fiber comprises providing acomplete preform that includes a plurality of holes in an outerdiameter, wherein the holes extend from a first end of a preform to asecond end of the preform, and forming at least one radiallyinwardly-extending slot within the preform such that the slot intersectsat least some of the holes, wherein the slot does not intersect at leastone hole. The method also includes establishing a first pressure in theholes intersected by the slot by introducing the first pressure to theslot, and establishing a second pressure in the at least one hole notintersected by the slot by introducing the second pressure to an end ofthe at least one hole not intersected by the slot. The method furtherincludes drawing the preform into a fiber while independentlycontrolling the first and second pressures.

WO 03/078338 discloses a method of making a preform for amicrostructured optical fiber using a sol-gel process. The describedpreform may comprise concentric tubes and a plurality of tubes and/orrods placed between the concentric tubes.

DESCRIPTION OF THE INVENTION

In an embodiment it is an object to provide a preform element for amicrostructured optical fiber preform which makes the production of thepreform relatively simple compared to prior art methods.

In an embodiment it is an object to provide a preform element for amicrostructured optical fiber preform which makes the production of thepreform economically beneficial compared to prior art methods.

In an embodiment it is an object to provide a preform element for amicrostructured optical fiber comprising a plurality of holes extendingalong the length of the fiber.

In an embodiment it is an object to provide a method of producing amicrostructured optical fiber which is relatively simple and offerslarge freedom in design of the fiber and where the diameter oflongitudinal holes can be controlled with a high accuracy.

These and other objects have been solved by the invention or embodimentsthereof as defined in the claims and as described herein below.

It has been found that the invention or embodiments thereof have anumber of additional advantages which will be clear to the skilledperson from the following description.

The phrase “radial distance” means distance determined in radialdirection perpendicular to the center axis. The phrase “radialdirection” is a direction from the center axis and radially outwards ora direction radially inwards towards the center axis. The term“substantially” should herein be taken to mean that ordinary productvariances and tolerances are comprised.

The terms “microstructured fibers” and “microstructured optical fibers”are used interchangeably and in this context are meant to cover fiberscomprising microstructures such as photonic crystal fibers, photonicband gap fibers, leaky channel fibers, holey fibers, etc. Themicrostructured optical fiber comprises a core region and a claddingregion surrounding the core where the cladding region may have severallayers and/or cladding sub regions with different refractive index, suchsub regions will usually be referred to as inner cladding region, outercladding region and optionally further regions, where inner and outerare determined in radial direction from the center axis of the opticalfiber or the preform or preform element. Where the terms core region andcladding region are used about a preform or a preform element, the termsrefers to the region the after the preform assembly comprising thepreform element has been drawn to a fiber. Unless otherwise noted, therefractive index refers to the average refractive index which is usuallycalculated separately for the core region and each cladding layersurrounding it and/or optionally for each cladding sub region. Acladding layer is defined as a layer with a thickness which layersurrounds the core region where the refractive index is substantiallyhomogeneous or where the layer has a base material with a substantiallyhomogeneous refractive index and a plurality of microstructures arrangedin a uniform pattern.

The term “about” is generally used to include what is within measurementuncertainties. The term “about” when used in ranges, should herein betaken to mean that what is within measurement uncertainties are includedin the range.

The term “longitudinal hole” means that the hole generally extendingalong the length of the preform ring element, such as along the holelength or along a section of the length. The term “hole” means“longitudinal hole” unless anything else is specified.

It should be emphasized that the term “comprises/comprising” when usedherein is to be interpreted as an open term, i.e. it should be taken tospecify the presence of specifically stated feature(s), such aselement(s), unit(s), integer(s), step(s) component(s) and combination(s)thereof, but does not preclude the presence or addition of one or moreother stated features.

Throughout the description or claims, the singular encompasses theplural unless otherwise specified or required by the context.

According to the invention it has been found to be highly advantageousto use a preform element according to the invention in the production ofa preform and a fiber drawn therefrom. The preform element may thus beused as a building block together with any additional preform element.Thereby a very flexible and cost effective production method ofmicrostructured optical fibers and preforms therefore has been provided.

In particular it has been found that by producing the preform elementaccording to an embodiment of the invention separately from the one ormore additional preform elements is very cost effective, because if thepreform element according to an embodiment of the invention is damagedduring the step comprising providing the intersecting slot, theadditional preform element(s) will not be damaged and may thus still beused.

The term “additional preform element” is used to mean an additionalpreform element relative to the preform element according to anembodiment of the invention, where the additional preform element may ormay not be a preform element according to an embodiment of theinvention.

The term “preform element” generally means a preform element accordingto an embodiment of the invention.

Further it has been found that the preform element ensure a desiredmethod of controlling the pressure within the longitudinal hole(s) ofthe preform element in that the pressure control may be provided in avery beneficial way and with a high accuracy via the slot whichintersects one or more of the longitudinal holes. Further due to thepreform element this slot may be provided prior to assembling thepreform element with additional preform elements of the preform. Thismay be much simpler since the preform element may be simpler to handleduring the performing of the intersecting slot than a final preformassembly and further the risk of damaging the preform assembly may bereduced.

The preform element has a length and a center axis along its length, afirst and second end defined by its length and an outer preform elementsurface. The preform element comprising a plurality of longitudinalstructures disposed to form longitudinal hole(s) in a backgroundmaterial. The preform element comprises at least one slot extending fromits outer preform element surface and intersecting at least one of thelongitudinal holes and wherein the at least one slot does not fullyintersect the preform element.

The intersecting slot is adapted for providing pressure control withinthe intersected holes during drawing of the preform assembly to a fiber.The preform element does not constitute a final preform but needs to beassembled with at least one additional preform element to form a preformassembly for which the optical fiber may be drawn.

As it will be explained further below the preform element mayadvantageously be a preform center element or a preform ring element,however it is not excluded that the preform element could form otherparts of a preform assembly, such as a half preform ring element andetc.

As explained in the background art, it is well known to control thepressure of holes during drawing of a preform to a microstructuredoptical fiber. For example the method disclosed in U.S. Pat. No.7,793,521 describes an advantageous way of controlling the hole pressureduring drawing. However, this method requires providing a radiallyinwardly extending slot in the final preform to intersect the holes thatare to be pressure controlled. Due to the high fragility of a finalpreform, it may frequently happen that the preform is damaged duringattempt to make such slot e.g. by cutting to deep or by fully breakingthe preform. In such case—in the method of U.S. Pat. No. 7,793,521—thewhole preform must be discarded.

In the present invention a slot e.g. a radially inwardly extending slotcan simply be made in the preform element before or after pre-drawing.At that stage the preform element is much simpler to handle and any riskof cutting too deep may be reduced. Further, even if the preform elementis damaged during cutting of the slot only this preform element or onlya section of it which may be cut off will be discarded and not the wholepreform. The preform element may be produced to be shelved for later useand due to the invention is very advantageous to have a stock of preformelements ready for use in the production of final preform assemblies.Thereby if a preform element is damaged during slot cutting it cansimply be replaced with a fresh preform element and thereby any delaymay be reduced or completely avoided.

Preferably the plurality of longitudinal structures are encaged inand/or forms part of the background material and extending in the lengthdirection of the preform element, wherein one or more of the pluralityof longitudinal structures form(s) the longitudinal hole(s). Thebackground material may for example be provided fully or partly by solidlongitudinal structures and/or by material of hollow longitudinalstructures, such as hollow tubes, e.g. capillary tubes.

The term “encaged in” is used to mean that the longitudinal structuresis mechanically locked or hold in position by elements, such as thecanes described below forming background material.

In an embodiment one or more of the plurality of longitudinal structuresare hollow tubes with the longitudinal holes and/or the longitudinalholes are longitudinal interspace holes formed between the longitudinalstructures, wherein the least one slot extending from its outer preformelement surface intersects at least one of the holes of the hollow tubesand/or at least one of the interspace holes.

The longitudinal structures and further parts of the preform element ifany may in principle be of any material from which an optical fiber canbe drawn. Suitable materials include any kind of glass, such as silicaglass, soft glass chalcogenide glass or any combinations thereof.

The longitudinal structure may in principle be any kind of longitudinalstructure which can be applied in optical fibers. The longitudinalstructure may in principle extend in any lengths of the preform element.Advantageously the preform ring element extends in substantially thewhole length of the preform element.

The longitudinal holes may be provided by using one or more hollow tubesand/or by arranging the longitudinal structures to form longitudinalinterspace holes between adjacent longitudinal structures.

In an embodiment the longitudinal structures comprises one or more solidstructures, such as rods.

In an embodiment the longitudinal structures comprises one or morehollow tubes, such a capillary tubes. Advantageously the hollow tubesare of glass, such as silica glass, soft glass chalcogenide glass or anycombinations thereof, optionally the hollow tube glass comprises one ormore dopants, such as one or more, index-changing dopants, such as Ge,B, F, P, Al and/or one or more active material dopants, such as therare-earth elements Er or Yb.

Advantageously the background material is glass, such as silica glass,soft glass chalcogenide glass or any combinations thereof, optionallythe background material comprises one or more dopants, such as one ormore, index-changing dopants, such as Ge, B, F, P, Al and/or one or moreactive material dopants, such as the rare-earth elements Er or Yb.

Generally it is desired that the preform element comprises a pluralityof longitudinal structures distributed in the preform element to providea desired optical function in the final microstructured optical fibere.g. to provide a desired effective refractive index in a part of thecladding such as a layer and/or a cladding sub region of themicrostructured optical fiber.

The longitudinal structures are preferably arranged to be substantiallyparallel to the center axis and preferably extend in substantially thewhole length of the preform element. In an embodiment the longitudinalstructures are arranged to helically surround the center axis. Suchpreform ring element is advantageous in the production ofchirally-coupled-core (CCC) fibers.

In an embodiment the hollow tubes each comprise a plurality oflongitudinal holes substantially parallel to the center axis of thepreform element, the plurality of holes of each hollow tube arepreferably arranged in a predetermined cross sectional patterncomprising at least one ring of holes and optionally a center hole.

In an embodiment each hollow tube comprises one single longitudinalhole, for example the hollow tube is in the form of a capillary tube.

In an embodiment the hollow tubes each comprise several longitudinalholes for example a plurality of substantially parallel holes extendingin length direction of the hollow tube. By applying hollow tubes withtwo or more holes it may be simpler to provide a microstructured opticalfiber from a preform comprising the preform element as it will bedescribed further below. In an embodiment the hollow tube has 3 or moreholes. In an embodiment the hollow tube has 6 or more holes, such as atleast 9 holes. The holes of the hollow tube are advantageously arrangedin a predetermined cross sectional pattern comprising at least one ringof holes and optionally a center hole.

The phrase “ring of holes” should herein be interpreted to mean a crosssectional ring shaped pattern of holes with intermediate materialbridges between holes separating adjacent holes from each other. Inother words, a “ring of holes” is—when seen in a cross-sectionalcut—composed of alternating holes and bridges. Each bridge has a minimumwidth determined as the shortest distance between two adjacent holes ofa ring of holes.

A hollow tube with two or more holes is also referred to as amulti-hollow tube.

In an embodiment the longitudinal structures comprises at least one ringof longitudinal structures.

The phrase “ring of longitudinal structures” should be interpreted tomean longitudinal structures in the form of solid structures, hollowtubes or a combination thereof arranged in a side by side configurationto form a ring.

In an embodiment the preform element has n fold rotational symmetry withrespect to rotations about its center axis, wherein n is up to 500, suchas up to 100, such as up to 50, such as from 2 to 20.

Advantageously the longitudinal structures extend in substantially theentire length of the preform element.

Advantageously, the preform element is at least partly fused prior toperforming the slot. Thereby the

In an embodiment at least one, such as at least all of the plurality oflongitudinal structures are hollow tubes and/or the preform elementcomprises one or more interspace holes, wherein the preform elementcomprises at least one slot extending from its outer preform elementsurface and intersecting at least one of the holes of the hollow tubesand/or at least one of the interspace holes such that the at least oneslot does not fully intersect the preform element. Preferably the atleast one slot is an annular slot intersecting a plurality, such as halfor substantially all of the holes of the hollow tubes and/or interspacehole.

In practice the slot may be as described in U.S. Pat. No. 7,793,521 withthe difference that the slot is performed in the preform element.

In an embodiment the at least one slot is in the form of a radiallyinward cut in the preform element from its outer preform elementsurface. The slot may be substantially perpendicular to the center axisor it may have an angle to the cross sectional plan perpendicular to thecenter axis e.g. in the form of an oblique annular cut e.g. with anangle up to about 45 degrees, such as up to about 25 degrees, such as upto about 15 degrees. In practice it is simpler to prepare the slot to besubstantially perpendicular to the center axis.

Advantageously the at least one slot is an annular slot intersecting aplurality of the holes of the hollow tubes and/or all of the interspaceholes, preferably, such as substantially all of the holes of the hollowtubes and/or all of the interspace holes of the preform cladding regionor the outer preform cladding region are intersected by said annularslot.

The preform element may comprise several slots e.g. one or more archshaped slot which together intersects the desired holes to beintersected. In an embodiment the preform element comprises a pluralityof arc shaped slots each having an arc shape relative to the outerpreform element surface of at least about 50 degrees such as from 80degrees to about 350 degrees, such as from about 150 degrees to about180 degrees. The plurality of slots may for example be axially displacedalong the preform element center axis.

By providing the preform element with several axially displaced slots,the slots may be provided to intersect relatively many holes whilesimultaneously ensure a high mechanically stability of the preformelement.

In an embodiment the preform element comprises two or more arc shapedslots which are arranged with a rotational displacement relative to eachother.

In an embodiment the preform element comprises two or more arc shapedslots arranged with a relative configuration as described 7,793,521.

The slot may in practice be positioned at any position preferably nottoo far from the first end of the preform element. The first end of thepreform element is thus the end at which the pressure control is adaptedto be provided and the second end of the preform element is the end atwhich the microstructured optical fiber is adapted to be drawn.

In an embodiment the at least one slot is positioned at a distance fromthe first end of the preform element which is up to 1/10 of the lengthof the preform element, the distance is preferably at least about 0.5 mmor longer, such as at least about 5 mm or longer, such as about 1 cm ormore. As mentioned, it is desired that the slot is not too far from thefirst end because the part of the preform element that can be drawn tomicrostructured optical fiber is at most from the second end to the slotand naturally it is not desired to waste unnecessarily lengths ofpreform element.

Advantageously the distance from the outer preform element surface to anradially outermost of said at least one intersected hole is less thanabout ⅛ of the preform element diameter, such as less than about 1/10 ofthe preform diameter, such as less than 1 cm, such as less than about0.5 cm, such as less than about 3 mm. By keeping the distance from theouter surface to the intersected hole(s) as short as possibly, the riskof damaging the preform element during the performing of the slot may behighly reduced. This embodiment is particular useful where the preformelement is adapted to be assembled with an additional preform element inthe form of an overcladding tube. The overcladding tube provides thepreform assembly as well as the final optical fiber with mechanicalstrength and for many optical fibers such an overcladding tube isrequired to ensure low leakage of light and further the overcladdingtube may be made of less expensive glass material with a lower puritythan the core region material and preferably also than the claddingmaterial immediately surrounding the core region. The overcladding tubemay e.g. be of silica or flour doped silica.

In an embodiment the intersected holes (originating from the hollowtubes or interspace holes) are closed at a position from the slot to thefirst end, such as at the first end of the preform and/or at theintersection towards the first end and/or a position there between.

In an embodiment the intersected holes are closed at an additional slotperformed for closing such holes. The intersected holes may be closed byusing a closing material, such as epoxy or similar sealing material andor the intersected holed may be closed by applying heat to collapse theholes.

As mentioned above the preform element may advantageously be a preformcenter element. The preform center element is adapted for be assembledwith at least one additional preform element surrounding the preformcenter element. The at least one additional preform element may be anadditional preform ring element such as a preform ring element accordingto an embodiment of the invention.

The preform center element comprises at least a preform core region anda preform cladding region surrounding the preform core region, whereinthe plurality of longitudinal structures forming the longitudinal holesare disposed in the preform cladding region. The holes may in principlebe arranged with cross-sectional configuration, preferably comprising apattern of holes. Advantageously the holes are arranged in across-sectional hexagonal or octagonal pattern.

In an embodiment the preform cladding region comprises an inner preformcladding region surrounding the preform core region and an outer preformcladding region surrounding the inner preform cladding region, whereinthe plurality of longitudinal structures forming the longitudinal holesare disposed in the outer preform cladding region. Thereby thelongitudinal holes in the outer preform cladding region may be subjectedto pressure control during drawing via the slot.

In an embodiment the plurality of longitudinal structures are disposedto form a ring of longitudinal holes in the preform cladding region.Such ring of longitudinal holes may for example be adapted for providingan air cladding in a fiber drawn from a preform comprising the preformelement. By subjecting the ring of holes to pressure control via theslot a highly accurate pressure control may be obtained and the achievedair filling factor of the air cladding may be desirably high.

As mentioned above the preform element may advantageously be a preformring element. The preform ring element is advantageously adapted forbeing assembled with at least one additional preform element comprisingat least an additional preform center element and optionally one or moreadditional preform ring elements and optionally an overcladding tube.

In an embodiment the preform ring element of an embodiment of theinvention comprises a ring shaped wall with a length and a center axisalong its length. The preform ring element has an outer preform elementsurface, an inner preform element surface and a first and second enddefined by its length. The preform ring element comprises a ring wallbackground material and a plurality of longitudinal structures encagedin the ring wall background material and extending in the lengthdirection of the preform.

One or more of the plurality of longitudinal structures formlongitudinal holes in the preform ring element. The preform ring elementcomprises at least one slot extending from its outer preform elementsurface and intersecting at least one of the longitudinal holes suchthat the at least one slot does not fully intersect the ring wall. Thephrases “length of the ring shaped wall” and “the length of the preformring element” are used interchangeably.

The preform ring element may be adapted to be assembled with a preformcenter element which comprises the core part of the final fiber preformfor being drawn to a microstructured optical fiber.

By producing the (additional) preform center element and the(additional) preform ring element of an embodiment of the inventionseparately a much simpler production can be obtained and the preformelement may be produced in relatively long lengths and be cut intodesired lengths and stored for later use with selected preform centerelements.

The preform ring element may advantageously be provided by arranging aninner hollow cane inside an outer hollow cane and arranging thelongitudinal structures between the inner and outer cane.

The ring wall background material may have any refractive index and bedoped or non-doped.

In an embodiment the ring wall background material comprises a ring wallinner background material e.g. provided by the inner cane and a ringwall outer background material e.g. provided by the outer cane, whichinner and outer background material may be equal or may differ from eachother. The inner and outer background material may for example differfrom each other with respect to refractive index, index-changingdopants, such as Ge, B, F, P, Al and/or active material dopant, such asthe rare-earth elements Er or Yb.

In an embodiment the inner and outer background material issubstantially free of active material dopant.

In an embodiment the inner ring wall inner background material is silicadoped with fluorine and the outer background material is non dopedsilica.

The elements—i.e. the element(s) forming the ring wall backgroundmaterial and the longitudinal structures—of the preform ring element mayin principle be of any material from which an optical fiber can bedrawn. Suitable materials include any kind of glass, such as silicaglass, soft glass chalcogenide glass or any combinations thereof.

The longitudinal structure encaged in the ring wall background materialmay in principle be any kind of longitudinal structure which can beapplied in optical fibers. The longitudinal structure may in principleextend in any lengths of the preform ring element. Advantageously thepreform ring element extends in substantially the whole length of thepreform ring element.

Generally it is desired that the preform ring element comprises aplurality of longitudinal structures distributed in the ring shaped wallof the preform ring element to provide a desired optical function in thefinal microstructured optical fiber e.g. to provide a desired effectiverefractive index in a part of the cladding such as a layer and/or acladding sub region of the microstructured optical fiber.

In an embodiment where the longitudinal structures are arranged in aring of longitudinal structures, the ring of longitudinal structurescomprises a ring of contacting longitudinal structures where thelongitudinal structures are arranged in a configuration in contact withadjacent longitudinal structures along their lengths. The ring oflongitudinal structures is alternatively a ring of non-contactinglongitudinal structures where the longitudinal structures are arrangedin a configuration where they are not in contact with adjacentlongitudinal structures along their lengths thereby forming interspaceholes.

The term “interspace hole” is herein used to designate a longitudinalhole provided between adjacent longitudinal structures.

Distance arrangements are advantageously arranged at two or morepositions along the length of the preform element for controlling thedistance of the longitudinal structures. In an embodiment at least onedistance arrangement is arranged at each end of the preform element,e.g. in the form of blind elements arranged between the longitudinalstructures to provide the desired distance between the longitudinalstructures to form the interspace holes extending in the longitudinallength direction of the preform element, preferably in the whole lengthof the preform element except for the blinds. The blinds may e.g. besections of silica rods. In principle the blinds may have any length,but advantageously the blinds are selected to be relatively short inorder not to waste unnecessary material. At the same time the blindsshould advantageously be sufficiently long to ensure high stability ofthe preform ring element. The length of the blinds are for example forexample up to 50% of the perform length. In practice the blinds may beas short as possible while still being sufficiently large to position asdistance elements.

In an embodiment the longitudinal structures encaged in the ring wallbackground material comprise a solid structure. The solid structure isadvantageously a solid structure having a refractive index differentfrom an index of the ring wall background material. The solid structuremay be of the same or of a different material than the ring wallbackground material, such as silica and it may e.g. be doped to have adifferent refractive index. In an embodiment the preform ring elementcomprises a ring of solid longitudinal structures, wherein at least someof the solid longitudinal structures have a refractive index differentfrom the refractive index of the ring wall background material.

The ring of solid structures comprises in an embodiment a ring ofnon-contacting solid structures where the solid structures are arrangedin a configuration where they are not in contact with adjacent solidstructures along their lengths thereby forming the interspace holes asdescribed above. A distance arrangement may advantageously be arrangedat two or more positions along the length of the preform ring elementfor controlling the distance between the adjacent solid structures andthereby defining the size of the longitudinal interspace holes.

In an embodiment where the preform ring element comprises a ring wallinner background material and a ring wall outer background material withdifferent refractive index, preferably at least some of the solidlongitudinal structures have a refractive index different from at leastone of the ring wall inner background material and the ring wall outerbackground material. The ring wall inner background material and thering wall outer background material may advantageously be provided by aninner hollow cane and an outer hollow cane as described further below.

In an embodiment the longitudinal structures encaged in the ring wallbackground material comprises at least one hollow tube comprising a holeextending along the length of the hollow tube and preferably along thelength of the preform ring element i.e. a longitudinal hole. The hollowtube(s) is/are preferably substantially parallel to the center axis ofthe preform ring element.

Preferably the preform element, such as the preform center element orthe preform ring element comprises a ring of hollow tubes.

The one or more holes of the hollow tubes preferably extendsubstantially in the whole length of the hollow tube e.g. with theexception that the holes may be closed at one or both of its ends and/orat the slot as described further below.

The phrase “ring of hollow tubes” should be interpreted to mean hollowtubes arranged in side by side configuration e.g. corresponding to thering shaped wall e.g. so that the ring of hollow tubes is concentricallywith the ring shaped wall or alternatively in an hexagonal or octagonalconfiguration.

The ring of hollow tubes comprises in an embodiment a ring of contactinghollow tubes where the hollow tubes are arranged in a configuration incontact with adjacent hollow tubes along their lengths. The ring ofhollow tubes is alternatively a ring of non-contacting hollow tubeswhere the hollow tubes are arranged in a configuration where they arenot in contact with adjacent hollow tubes along their lengths therebyforming longitudinal interspace holes. A distance arrangement e.g. asdescribed above may advantageously be arranged at two or more positionsalong the length of the preform ring element for controlling thedistance of the hollow tubes and thereby defining the size of thelongitudinal interspace holes.

Generally the longitudinal holes may have equal or differentcross-sectional shape.

In the embodiment where the preform element comprises holes originatingfrom the holes of the hollow tubes as well as interspace holes, thehollow tube holes and the interspace holes may be equal or different insize and cross-sectional shape. Thereby a large design freedom may beobtained.

In an embodiment the preform ring element comprises hollow tubes, suchas multi hollow tubes and the hollow tubes may be of the same or adifferent material than the ring wall background material or materialse.g. as the materials discussed above.

The longitudinal structures may in principle have any cross-sectionalperiphery, such as round, square or with any number of angles. Where thepreform element is fused and/or pre-drawn as described below, thelongitudinal structures may be deformed to have a cross-sectionalperiphery which is approximately square or rectangular where the preformelement is a preform ring element and approximately square hexagonal oroctagonal where the preform element is a preform center element.

In an embodiment the longitudinal structures are arranged in a ring oflongitudinal structures and the respective longitudinal structures areapproximately rectangular with a large cross dimension and a short crossdimension and preferably the longitudinal structures are arranged withtheir respective large cross dimensions substantially in radialdirection.

In an embodiment where the preform element is a preform ring element,the ring wall may have any thickness determined from the inner preformelement surface to the outer preform element surface. In practice thethickness of the ring wall may be different or equal along the annularextension of the ring shape. Preferably the ring wall thickness issubstantially uniform along its length, which makes it simpler toassemble the preform ring element with one or more additional preformelements to a final preform assembly and to draw a microstructuredoptical fiber from the preform assembly.

In an embodiment the ring wall has a substantially identical thicknessalong its annular extension at least in a cross section, preferably inits whole length, more preferably the preform ring element issubstantially circular symmetrical in its center axis with respect tothickness.

The ring wall has an average inner diameter (ID) defined by the innerpreform element surface and an average outer diameter (OD) defined bythe outer preform element surface.

The average inner diameter (ID) and average outer diameter (OD) may haveany size where OD>ID. In practice, average inner diameter (ID) shouldadvantageously not be too small. If the average inner diameter (ID) ofthe preform ring element to be assembled with a preform center elementhas a too large average inner diameter (ID), the preform ring elementmay be pre-drawn to obtain the desired average inner diameter (ID).

In general it is desired that average inner diameter (ID) is at leastabout 1 mm, such as at least about 2 mm, such as at least about 3 mm,such as at least about 5 mm, such as at least about 8 mm.

The average inner diameter (ID) and the average outer diameter (OD) areadvantageously substantially constant along the length of the preformring element, preferably the thickness of the preform ring element isdefined by 0.5 times OD minus ID is from about 0.3 mm to about 2 cm,such as from about 1 mm to about 1 cm, such as from about 3 mm to about6 mm.

In practice, the relative size of ID and OD is preferably selected suchthat ID/OD in at least a cross section of the preform ring element isfrom about 0.3 to about 0.95, such as from about 0.7 to about 0.9.

In an embodiment the ring wall comprises an inner hollow cane, an outerhollow cane and a plurality of longitudinal structures encaged betweenthe inner cane and the outer cane.

In an embodiment the longitudinal structures are encaged by being heldin place in the ring shaped wall background material, such as betweenthe inner hollow cane and the outer hollow cane. To ensure a safehandling of the preform ring element without any risk of thelongitudinal structures slipping out of the encaging, the longitudinalstructures may be held in position by holding arrangement positions atone or both ends of the preform ring element and/or the parts of thepreform ring element e.g. the longitudinal structures and the ringshaped wall background material may be fused together. A holdingarrangement may simultaneously serve to close optional holes and/orserve as a distance arrangement as described above.

The inner hollow cane and the outer hollow cane may have equal ordifferent cane wall thickness and they may be of equal or differentmaterial and/or have equal or different refractive index.

In an embodiment the inner hollow cane has an inner cane wall thicknessand the outer cane has an outer cane wall thickness, wherein the innercane wall thickness and the outer cane wall thickness independently ofeach other are from about 1 mm to about 1 cm, such as from about 2 mm toabout 5 mm.

The inner hollow cane has an inner cane average inner diameter (ICID)and an inner cane average outer diameter (ICOD) and the outer hollowcane has an outer cane average inner diameter (OCID) and an outer caneaverage outer diameter (OCOD)

The ICID/ICOD and the OCID/OCOD are preferably, independently of eachother from about 0.3 to about 0.95, such as from about 0.7 to about 0.9.

As explained the preform element may be pre-drawn to a desired innerdiameter before being assembled with additional preform element(s).Therefore, where the preform element is a preform ring element therelative size of the inner and outer diameters of the preform ringelement as well as of the inner and outer cane is more important thatthe actual size and advantageously the preform ring element is initiallyproduced with larger inner diameter than actually expected to berequired for use in a final preform.

A preform element to be assembled with additional preform element(s) mayin principle have any length which is handleable and generally it isdesired that the preform element has a length of from about 50 cm toabout 150 cm in its state for being assembled e.g. after pre-drawing. Inan embodiment the preform element is initially produced to have a lengthup to about 1.5 m, such as up to about 1 m, such as up to about 0.5 m.Thereafter the preform element may be pre-drawn to have a larger length,where after or during the pre-drawing it is cut into several preformelements of shorter lengths and/or the initially produced preformelement is cut into several preform elements of shorter lengths e.g.50-150 cm each, after the respective shorter length preform elements maybe pre-drawn prior to being assembled with additional preform elementsfor a final preform assembly.

In an embodiment the preform ring element has an inner diameter of atleast about 5 mm, such as at least about 10 mm, such as from about 15 toabout 5 cm or even 10 cm.

In an embodiment the ring wall of the preform ring element has fullrotational symmetry with respect to thickness. In practice it is simplerto handle such preform ring elements since they are simpler to assemblewith additional preform element(s).

Alternatively the ring wall of preform ring element may have n foldrotational symmetry in its center axis with respect to thickness,wherein n is up to 500, such as up to 100, such as up to 50, such asfrom 2 to 20. In practice it is desired that the ring wall of thepreform ring element with n fold rotational symmetry in its center axishas at most 10 fold symmetry, such as 6 to 9 fold symmetry.

Advantageously the preform ring element comprises a plurality oflongitudinal structures encaged in the ring wall background material,wherein the longitudinal structures are preferably extending insubstantially the entire length of the preform ring element.

In an embodiment the longitudinal structures encaged in the ring wallbackground material are equal or different in material and/or shapeand/or size.

Advantageously the preform ring element comprises one or more rings oflongitudinal structures, where each ring of longitudinal structures hasequal outer dimension, thereby making it relatively simple to assemblethe preform ring element e.g. by encaging the ring of longitudinalstructures between an inner and an outer hollow cane. The longitudinalstructures of a ring of longitudinal structures may optionally differfrom each other. In an embodiment the longitudinal structures of a ringof longitudinal structures comprise alternating solid structure andhollow tubes. In an embodiment the longitudinal structures of a ring oflongitudinal structures comprises hollow tubes with alternating largeand small hole diameters.

In an embodiment the plurality of longitudinal structures are arrangedin a side by side configuration along the annular extension of the ringshaped wall to form at least one ring of longitudinal structures, suchas two rings of longitudinal structures, such as three rings oflongitudinal structures. The option of having more than one ring oflongitudinal structures provides even more freedom for preparing finalpreform assemblies in a simple way and the preform ring element may thusbe used to provide a large part of the cladding region, such as an innercladding, an outer cladding or a part thereof or even all of thecladding of the final microstructured optical fiber drawn from a preformcomprising the preform ring element.

In an embodiment where the preform ring element comprises two or morerings of longitudinal structures the two or more rings of longitudinalstructures are advantageously rotationally offset, which makes itsimpler to arrange the longitudinal structures between an outer and aninner hollow cane. For simple assembling the longitudinal structures mayhave an angular outer shape as described above, such as square orrectangular or with a single planar side.

In an embodiment the preform ring element comprises at most two rings oflongitudinal structures.

The present invention is particularly beneficial where the preformelement comprises at least one hollow tube and/or at least oneinterspace hole. Due to the preform element such holes of hollow tubesand such interspace holes can in a simple way be controlled duringdrawing of the final preform comprising such preform ring element to anoptical fiber.

The preform element is advantageously obtained by the method describedin the following.

The invention also comprises a method of producing a preform elemente.g. as described above.

The method comprises providing a plurality of longitudinal structuresand disposing the longitudinal structures to form longitudinal hole(s)in a background material the method comprises providing at least oneslot extending from the outer preform element surface and intersectingat least one of the longitudinal holes and wherein the at least one slotdoes not fully intersect the preform element.

The longitudinal structures may be as described above and thelongitudinal holes may be provided by hollow tubes or as interspaceholes between longitudinal structures.

Advantageously the method comprises arranging the longitudinalstructures to provide the plurality of longitudinal holes substantiallyparallel to the center axis of the preform element.

Preferably the method comprising providing the at least one slot is inthe form of a radially inwards cut in the preform element from its outerpreform element surface, wherein the radially inwards cut issubstantially perpendicular to the center axis. The cut may e.g. be inthe form of an oblique annular cut e.g. with an angle up to about 45degrees, such as up to about 25 degrees, such as up to about 15 degrees.In practice it is simpler to prepare the slot to be substantiallyperpendicular to the center axis

In an embodiment the method comprises providing several slots e.g. oneor more arch shaped slot which together intersects the desired holes tobe intersected.

In an embodiment the method comprises providing a plurality of slots byproviding a plurality of arc shaped radially inwards cuts each having anarc shape relative to the outer preform element surface of at leastabout 50 degrees such as from 80 degrees to about 350 degrees, such asfrom about 150 degrees to about 180 degrees. The plurality of slots mayfor example be axially displaced along the preform element center axis.

To ensure high stability even where the cuts are to intersect relativelymany holes the plurality of radially inwards cuts may advantageously bedisplaced along the axis of the preform element and preferablyrotationally off set relative to each other.

Advantageously the at least one slot is provided at a distance from thefirst end of the preform element which is preferably up to 1/10 of thelength of the preform element, the distance is preferably at least about0.5 mm or longer, at least about 5 mm or longer, such as about 1 cm toabout 20 cm.

The method may further comprise closing the intersected holes at aposition from the first end of the preform to the intersection, such asat the first end of the preform and/or at the intersection towards thefirst end of the preform and/or at an additional slot intersecting theholes, the intersected holes are preferably closed by collapsing and/orby filling a solid material into the holes as described above.

In an embodiment the method comprises stacking the longitudinalstructures and optionally arranging the stacked longitudinal structuresin an outer cane. The method of stacking longitudinal structures arewell known in the art and will thus not be described further herein.

In an embodiment where the preform element is a preform center elementthe method comprises stacking the longitudinal structures to form atleast the preform core region and the preform cladding regionsurrounding the preform core region, wherein the plurality oflongitudinal structures forming the longitudinal holes are disposed inthe preform cladding region. The longitudinal structures may inprinciple be stacked to dispose the holes with any cross sectionalstructures, however it is preferred that the holes are disposed in across-sectional hexagonal or octagonal pattern.

In an embodiment the method comprises arranging the longitudinalstructures to form the preform cladding region such that is comprises aninner preform cladding region surrounding the preform core region and anouter preform cladding region surrounding the inner preform claddingregion. Thereby the slot may be performed to intersect the holes of theouter cladding region.

In an embodiment the method comprises disposing the plurality oflongitudinal structures to form a ring of longitudinal holes in thepreform cladding region. The ring of longitudinal holes may be adaptedfor providing an air cladding in a fiber drawn from a preform comprisingthe preform element.

In an embodiment the method comprises performing the at least one slotin the form of an annular slot intersecting a plurality of the holes ofthe hollow tubes and/or all of the interspace holes. Where the preformelement is a preform ring element it is advantageous that substantiallyall of the holes of the hollow tubes and/or all of the interspace holesof the preform ring element are intersected by the annular slot. Wherethe preform element is a preform center element it is preferred that atleast the all of the holes of the hollow tubes and/or all of theinterspace holes of the preform cladding region or the outer preformcladding region

In an embodiment where the preform element is a preform ring element,the method comprises providing an outer ring wall section, an inner ringwall section and a plurality of longitudinal structures and encaging thelongitudinal structures in between the outer ring wall section and theinner ring wall section to form longitudinal holes and providing atleast one slot extending through the outer ring wall section andintersecting at least one of said longitudinal holes. As mentioned abovethe preform ring element may comprise additional element such as holdingarrangement(s) and/or distance arrangement(s).

The outer ring wall section may have an outer surface providing theouter preform element surface and the method comprises performing the atleast one slot to extend from the outer preform element surface andintersecting said longitudinal hole(s). The inner and outer ring wallsections as well as the longitudinal structures may be as describedabove.

Advantageously the outer ring wall section is an outer hollow cane andthe inner ring wall section is an inner hollow cane.

Preferably the encaging of the longitudinal structures in between theouter ring wall section and the inner ring wall section comprisesassembling the outer ring wall section and the inner ring wall sectionwith the longitudinal structures in between, optionally arrangingholding arrangement(s) and/or distance arrangement(s), and fusing theouter ring wall section and the inner ring wall section together via thelongitudinal structures.

In an embodiment the assembling comprising arranging the inner wallsection with its outer surface of the inner ring wall to face the innersurface of the outer ring wall and arranging the one longitudinalstructures between the outer surface of the inner ring wall and theinner surface of the outer ring wall, such as arranging a plurality oflongitudinal structures between the outer surface of the inner ring wallin a ring of longitudinal structures optionally with distancearrangements to provide one or more interspace holes between adjacentlongitudinal structures.

In an embodiment the inner and outer ring wall sections are hollow canesand the assembling comprises inserting the inner ring wall section intothe outer ring wall section and arranging a plurality of thelongitudinal structures between the inner ring wall section and theouter ring wall section optionally with distance arrangements to provideone or more interspace holes between adjacent hollow canes.

In an embodiment the method comprising arranging the longitudinalstructures in a side by side configuration to surround the inner ringwall section, to form at least one ring of longitudinal structures, suchas two rings of longitudinal structures, such as three rings oflongitudinal structures.

Advantageously one or more of the plurality of longitudinal structuresare hollow tubes with the longitudinal holes and/or the longitudinalholes are longitudinal interspace holes between the longitudinalstructures, wherein the least one slot is provided to intersects atleast one of the holes of the hollow tubes and/or at least one of theinterspace holes. Preferably all of the longitudinal structures arehollow tubes, optionally including one or more tubes with a plurality oflongitudinal holes (multi hollow tubes) substantially parallel to thecenter axis of the preform element.

In an embodiment the longitudinal structures are arranged in aconfiguration where the respective longitudinal structures are incontact with adjacent longitudinal structures along their lengths.

In an embodiment the longitudinal structures are arranged in a ring oflongitudinal structures in a configuration where the respectivelongitudinal structures are not in contact with adjacent longitudinalstructures thereby forming the longitudinal holes in the form ofinterspace longitudinal holes as described above. Preferably the methodcomprises arranging a distance arrangement at two or more positionsalong the length of the preform ring element for controlling thedistance between the longitudinal structures. Thereby the distancearrangements may also be applied for controlling the size of theinterspace holes of the longitudinal structures.

In an embodiment the preform element is a preform ring element and aplurality of the longitudinal structures are hollow tubes, eachcomprising at least one hole and/or the preform comprises a ring oflongitudinal structures with interspace holes, the method comprisesproviding at least one slot extending from the outer preform elementsurface and intersecting a plurality of the holes and/or interspaceholes, wherein the at least one slot does not fully intersect the ringwall. Preferably the at least one slot is an annular slot intersecting aplurality, such as substantially all of the holes and/or interspaceholes. The slot may be as described above.

Advantageously, the preform element is at least partly fused prior toperforming the slot in order to ensure stability of the preform elementduring the cutting process. The term “at least partly fused” areintended to mean that the longitudinal structures are fused to adjacentlongitudinal structures and or to a structure forming the backgroundmaterial, e.g. the inner and/or the outer hollow cane in the case pf apreform ring element.

The invention also comprises a method of producing a microstructuredoptical fiber from a preform assembly comprising a preform element asdescribed above and at least one additional preform element.

In an embodiment the method of producing a microstructured fibercomprises a first step of assembling a preform to a preform assembly anda second step of drawing the microstructured optical fiber from thepreform assembly while controlling the pressure within the intersectedholes of the preform element.

Advantageously the method comprises subjecting the longitudinal holes ofsaid preform element intersected by the slot to a preform ring elementcontrolled pressure during the drawing, wherein the preform ring elementcontrolled pressure is applied via said slot.

In an embodiment the at least one additional preform element is anovercladding tube with a length and a first and a second end and a boreand the preform assembling comprises disposing said preform element intosaid bore of said overcladding tube to form a preformelement-overcladding tube interface, wherein the preform assembly has afirst end section comprising said first end of said preform element andsaid first end of said overcladding tube.

The phrase “preform element-overcladding tube interface” is used todesignate an interface between a preform element and an overcladdingtube, where the preform element optionally is specified to be a preformring element or a preform center element.

The phrase “preform ring element-preform center element interface” isused to designate an interface between a preform ring element and apreform center element wherein at least one of the preform elements isaccording to an embodiment of the invention and the other one of thepreform elements may be an additional preform element.

Together the interfaces discussed above are referred to as “elementsinterface(s)” and each of them is referred to as an elements interface.An elements interface may have an interface void in the form of a voidbetween the preform element and additional preform element forming theelements interface.

The overcladding tube may advantageously be of silica optionally dopede.g. with fluoride.

In an embodiment the preform element is a preform center element.

In an embodiment the preform element is a preform ring element.

In an embodiment the method comprises providing a preform ring elementaccording to an embodiment of the invention and arranging the ringshaped wall to surround an additional preform center element to form apreform ring element-preform center element interface. The additionalpreform center element and the preform ring element are assembled suchthat the preform assembly has a first end section comprising the firstend of the preform center element and the first end of the preform ringelement.

In an embodiment the method of producing a microstructured fibercomprises assembling a preform, and drawing the preform assembly to amicrostructured fiber, wherein the assembling of the preform comprisesproviding an additional preform center element with a length and a firstand a second end, an overcladding tube with a length and a first and asecond end and a bore and at least one preform ring element e.g. asdescribed above

-   -   arranging the ring shaped wall to surround the preform center        element to form a preform ring element-preform center element        interface and    -   arranging the ring shaped wall into the bore of the overcladding        tube to form a preform ring element-overcladding tube interface,        wherein the preform assembly has a first end section comprising        the first end of the preform center element, the first end of        the preform ring element and the first end of the overcladding        tube.

The (additional) preform center element usually will comprise thematerial that will provide the core region and normally a part of thecladding region of the microstructured optical fiber. In an embodimentthe (additional) preform center element is a hollow preform centerelement e.g. for providing a hollow fiber.

The (additional) preform center element may comprise any materials whichare usually used in prior art preforms.

The terms “core” and “core region” are used interchangeably and the term“cladding” and “cladding region” are used interchangeably.

In an embodiment the method comprises providing a preform center elementaccording to an embodiment of the invention and arranging additionalpreform ring element to surround the preform center element. The preformcenter element and the additional preform ring element are assembledsuch that the preform assembly has a first end section comprising thefirst end of the preform center element and the first end of the preformring element.

Prior to assembling of the preform the preform element may be pre-drawnto have a desired outer diameter and/or in the case of a preform ringelement to have a desired inner diameter relative to the outer diameterof the additional preform center element—e.g. an inner preform ringelement diameter slightly larger than the outer diameter of theadditional preform center element. Where there are several preform ringelements to be assembled to the preform assembly these two or morepreform ring elements may be pre-drawn simultaneously or separately.

The slot may be provided prior to or after the pre-drawing.

In an embodiment the assembling of the preform comprises providing twoor more preform ring elements of an embodiment of the invention andarranging the preform ring elements coaxially with the additionalpreform center element radially innermost.

In an embodiment the assembling of the preform comprises providing anadditional blind preform ring element without longitudinal structuresand arranging the preform ring element(s) of an embodiment of theinvention and the blind preform ring element(s) coaxially with theadditional preform center element radially innermost.

In an embodiment the assembling of the preform comprises providing apreform center element of an embodiment of the invention and anadditional preform ring element and arranging the additional preformring element coaxially with the preform center element radiallyinnermost.

The element may be partly or fully fused prior to drawing, but it maynot be necessary to fuse the elements provided that the various elementsare fully encaged or locked in the overcladding tube.

After assembling of the preform, the preform assembly is drawn to themicrostructured optical fiber e.g. in a drawing tower. Advantageouslythe preform is held at a first end section near its first end and themicrostructured optical fiber is drawn from the second end of thepreform at an increased temperature, e.g. as known from prior art fiberdrawing methods.

In an embodiment at least one elements interface is subjected to areduced pressure relative to atmospheric pressure during at least a partof the drawing of the preform assembly to a microstructured fiber.

In an embodiment comprising a preform ring element at least one of thepreform ring element-preform center element interface and the preformring element-overcladding tube-interface are subjected to a reducedpressure relative to atmospheric pressure during at least a part of thedrawing of the preform assembly to a microstructured fiber.

The reduced pressure at the elements interface(s) ensures that no air(or other gas) is captured in the respective elements interfaces to formundesired bubbles in the final microstructured optical fiber and thepreform element is fully fused to the additional preform element(s).

In an embodiment the preform ring element is fully fused to the(additional) preform center element and/or the preform ring element isfully fused to the overcladding tube in the drawn microstructuredoptical fiber.

The reduced pressure is advantageously a selectable pressure to beselected by the fiber manufacturer.

In an embodiment the (additional) preform center element comprises atleast one hole, such as a hole originating from a hollow tube or ainterspace hole, extending along its length, preferably from the firstend of the (additional) preform center element and substantially to asecond end of the (additional) preform center element, the methodcomprising subjecting the at least one hole of the (additional) preformcenter element to a preform center controlled pressure during thedrawing.

Where the preform center element is according to an embodiment of theinvention the pressure control is advantageously provided via the slotin the preform center element.

The preform center controlled pressure is selected to control the finalsize of the hole in the microstructured optical fiber. Where the preformcenter controlled pressure is a reduced pressure relative to atmosphericpressure, the hole may be fully or partly collapsed in the drawnmicrostructured optical fiber. Where the preform center controlledpressure is atmospheric pressure or is an increased pressure relative toatmospheric pressure, the size of the hole may be adjusted as it isknown in the art.

In an embodiment the (additional) preform center element comprises atleast a first category hole and at least a second category holeextending in length direction, such as from the first and to the secondend of the (additional) preform center element, the method comprisingsubjecting the at least one first category hole to a first preformcenter controlled pressure and subjecting the at least one secondcategory hole to a second preform center controlled pressure during thedrawing.

In an embodiment the pressure control of the (additional) preform centerelement hole(s) or of the radially innermost of the first and the secondcategory holes may be provided as described in U.S. Pat. No. 8,215,129and/or U.S. Pat. No. 6,954,574. Where the preform center element isaccording to an embodiment of the invention the pressure control of theradially outermost of the first and the second category holes isadvantageously provided via the slot in the preform center element.

In an embodiment the preform element comprises at least one hollow tubewith a hole and/or at least one interspace hole extending in lengthdirection, such as from the first end of the preform element to thesecond end of the preform element and the method comprising subjectingthe at least one hole of the preform element to a preform elementcontrolled pressure during the drawing.

Advantageously the holes intersected by the slot are subjected to thepreform element controlled pressure during the drawing via the slot.

The preform element controlled pressure is selected to control the finalsize of the intersected hole originating from the preform element in thedrawn microstructured optical fiber. The preform element controlledpressure may be a reduced pressure relative to atmospheric pressure,atmospheric pressure or an increased pressure relative to atmosphericpressure to adjust the size of the hole in the drawn microstructuredoptical fiber.

Preferably the preform element controlled pressure is controlledindependently of at least one other controlled pressure during thedrawing, such as the preform center controlled pressure subjected toholes of an additional preform center element.

Advantageously the at least one slot is positioned at an end section ofthe preform ring element protruding beyond the overcladding tube in theassembled preform. Thereby the preform element controlled pressure canbe applied at the end section not covered by the overcladding tube viathe slot. The slot is preferably positioned at a distance from the firstend of the preform element which is up to 1/10 of the length of thepreform ring element, the distance is preferably at least about 5 mm orlonger, such as about 1 cm to about 20 cm as described above.

In an embodiment the first end of the (additional) preform centerelement, the first end of the (additional) preform ring element and thefirst end of the overcladding tube are sequentially offset at thepreform end section, such that the end section of the (additional)preform ring element protrudes beyond the end of the overcladding tubeand preferably such that an end section of the (additional) preformcenter element protrudes beyond the end of the (additional) preform ringelement. At least one and optionally both of the (additional) preformcenter element and the (additional) preform ring element is according toan embodiment of the invention.

By this arrangement it is convenient to apply a reduced pressure to therespective elements interface as well as a preform element controlledpressure via the slot of the preform center element and/or the preformring element and optionally an additional preform center controlledpressure applied at the end of the (additional) preform center element.The respective pressure controls may be applied at separate sectionsalong the end section of the preform assembly with sequentially offsetpreform center element first end, preform ring element first end andovercladding tube first end, preferably with seals between the separatesections.

In an embodiment the one or more pressure controls are provided byenclosing the end section of the preform assembly by a pressure controlcap comprising two or more pressure control sections and applying sealsto seal off independently pressure controlled holes, comprising at leasta pair of slot seals on opposite side of the slot. Preferably theintersected holes are closed at a position from the first end of thepreform element to the intersection slot, such as at the first end ofthe preform element and/or at the intersection towards the first endand/or at an additional intersecting slot. Each of the pair of slotseals is preferably forming an annular seal between the pressure controlcap and the outer preform element surface of the end section of thepreform element.

Preferably the drawing of the preform assembly comprising drawing thepreform assembly to the microstructured optical fiber in a drawingtower, wherein the preform assembly is subjected to at least onepressure control at its first end section, preferably two or moreindependently controlled pressure controls.

In an embodiment the drawing of the preform assembly comprising drawingthe preform assembly from its second end section at an increasetemperature.

The invention also comprises a microstructured optical fiber obtainableby the method described above.

All features of the inventions and embodiments of the invention asdescribed above including ranges and preferred ranges can be combined invarious ways within the scope of the invention, unless there arespecific reasons not to combine such features.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and/or additional objects, features and advantages of thepresent invention will be further elucidated by the followingillustrative and non-limiting description of embodiments of the presentinvention, with reference to the appended drawings.

FIG. 1 is a cross-sectional view of an embodiment of a preform ringelement of an embodiment of the invention with solid structures and/orhollow canes.

FIG. 2a is a cross-sectional view of an embodiment of a preform ringelement of an embodiment of the invention with hollow tubes.

FIG. 2b is a section of an image of a cross-sectional view of anembodiment of a preform ring element corresponding to the embodiment ofFIG. 2a where the preform ring element has been fused and pre-drawn.

FIG. 3 is a cross-sectional view of an embodiment of a preform ringelement of an embodiment of the invention with interspace holes.

FIG. 4a is a cross-sectional view of an embodiment of a preform ringelement of an embodiment of the invention with multi-hollow tubes.

FIG. 4b is a section on an image of a cross-sectional view of anembodiment of a preform ring element corresponding to the embodiment ofFIG. 4a where the preform ring element has been fused and pre-drawn.

FIG. 5a is a cross-sectional view of a multi-hollow tube with 7 holes.

FIG. 5b is a cross-sectional view of a multi-hollow tube with 19 holes.

FIG. 6 is a side view of an end section of a preform comprising apreform ring element with a slot.

FIG. 7a is a schematic illustration of an end section of a preformassembly comprising a preform ring element mounted with a pressurecontrolling arrangement.

FIG. 7b is a schematic illustration of an end section of a preformassembly comprising a preform ring element and a preform element mountedwith a pressure controlling arrangement.

FIGS. 8, 9, 10 and 11 are cross-sectional views of assembled preforms,each comprising a preform ring element.

FIGS. 12, 13 and 14 are cross-sectional views of drawn microstructuredoptical fibers from preform assemblies, each comprising a preform ringelement according to an embodiment of the invention.

FIG. 15 is a cross-sectional view of a preform center element accordingto an embodiment of the invention.

FIG. 16 is a cross-sectional view of a preform center element accordingto another embodiment of the invention.

FIGS. 17 and 18 are cross-sectional views of drawn microstructuredoptical fibers from preform assemblies, each comprising a preform centerelement according to an embodiment of the invention.

The figures are schematic and may be simplified for clarity. Throughout,the same reference numerals are used for identical or correspondingparts.

Further scope of applicability of the present invention will becomeapparent from the description given hereinafter. However, it should beunderstood that the detailed description and specific examples, whileindicating preferred embodiments of the invention, are given by way ofillustration only, since various changes and modifications within thespirit and scope of the invention will become apparent to those skilledin the art from this detailed description.

The preform ring element shown in FIG. 1 comprises a ring shaped wall 1with a length which is perpendicular to the cross-sectional view and acenter axis C. The ring shaped wall 1 has an outer preform elementsurface OS defining the outer diameter OD and an inner preform elementsurface IS defining the inner diameter ID.

The preform ring element comprises a ring wall background materialprovided by an inner hollow cane 2 and an outer hollow cane 3 and aplurality of longitudinal structures 4, wherein at least one is a hollowtube encaged in the ring wall background material and extending in thelength direction of the preform ring element. A not shown slot isprovided from the outer surface OS and intersecting at least one hole ofthe hollow tube of the longitudinal structures 4.

The inner cane 2 and outer cane 3 may be as described above. In theshown embodiment the inner hollow cane 2 has a thinner wall thicknessthan the outer hollow cane 3 and advantageously the inner hollow canemay be fluorine doped. The outer preform element surface OS is providedby the outer surface of the outer hollow cane 3 and the hollow outercane 3 has an inner surface ISO. The inner preform element surface isprovided by the inner surface of the inner hollow cane 2 and the innerhollow cane 2 further has an outer surface OSI.

The longitudinal structures 4 comprise at least one hollow tube and theremaining may be solid structures and/or hollow tubes. Preferably thelongitudinal structures 4 are hollow tubes e.g. in the form of capillarytubes.

The longitudinal structures 4 are arranged in a side by sideconfiguration along the ring shape of the ring shaped wall to form aring of longitudinal structures.

The preform ring element shown in FIGS. 2a and 2b comprises a ringshaped wall 11 with a length which is perpendicular to thecross-sectional view and a center axis The ring shaped wall 11 has anouter preform element surface OS defining the outer diameter OD and aninner preform element surface IS defining the inner diameter ID. Thepreform ring element comprises a ring wall background material providedby an inner hollow cane 12 and an outer hollow cane 13 and a pluralityof longitudinal structures 14 encaged in the ring wall backgroundmaterial and extending in the length direction of the preform ringelement.

The longitudinal structures 14 are in the form of hollow tubes eachcomprising a longitudinal hole 14 a. The hollow tubes 14 are encagedbetween the inner hollow cane 12 and the outer hollow cane 13. A notshown slot is provided from the outer surface OS and intersecting atleast one hole of the hollow tubes 14.

In FIG. 2b the preform ring element has been fused and pre-drawn and itcan be seen that interstices between the hollow tubes 14 andrespectively the inner hollow cane 12 and the outer hollow cane 13 havebeen removed and further the hollow tubes have been deformed to besubstantially square shaped.

The preform ring element shown in FIG. 3 comprises a ring shaped wall 21with a length which is perpendicular to the cross-sectional view and acenter axis The ring shaped wall 21 has an outer preform element surfaceOS defining the outer diameter OD and an inner preform element surfaceIS defining the inner diameter ID.

The preform ring element comprises a ring wall background materialprovided by an inner hollow cane 22 and an outer hollow cane 23 and aplurality of longitudinal structures 24 encaged in the ring wallbackground material and extending in the length direction of the preformring element.

The longitudinal structures 24 are solid structures and/or hollow canes.Preferably the longitudinal structures 24 are hollow tubes e.g. in theform of capillary tubes.

The longitudinal structures 24 are encaged between the inner hollow cane22 and the outer hollow cane 23.

The longitudinal structures 24 are arranged in a ring of non-contactinglongitudinal structures where the longitudinal structures are arrangedin a configuration where they are not in contact with adjacentlongitudinal structures along their lengths thereby forming interspaceholes 25. A not shown slot is provided from the preform element outersurface OS and intersecting at least one hole of the interspace holes25.

To hold the longitudinal structures in position, not shown distancearrangements are advantageously arranged at two or more positions alongthe length of the preform ring element as described above.

The preform ring element shown in FIGS. 4a and 4b comprises a ringshaped wall with a length which is perpendicular to the cross-sectionalview and a center axis The ring shaped wall has an outer preform elementsurface OS defining the outer diameter OD and an inner preform elementsurface IS defining the inner diameter ID. The preform ring elementcomprises a ring wall background material provided by an inner hollowcane 32 and an outer hollow cane 33 and a plurality of longitudinalstructures in the form of multi-hollow tubes 34 encaged in the ring wallbackground material and extending in the length direction of the preformring element.

The multi-hollow tubes 34 each comprise a plurality of longitudinalholes extending in the length direction thereof.

Multi-hollow tubes 34 are very advantageous to use in the preform ringelement or in a preform center element where a slot is to intersect atleast some of the holes, because in such situation it may be sufficientto intersect some of the holes while still obtaining a desired airfraction, which air fraction can be controlled by the preform elementcontrolled pressure during drawing of the microstructured optical fiber.

In FIG. 4b the preform ring element has been fused and pre-drawn and itcan be seen that interstices between the multi-hollow tubes 34 andrespectively the inner hollow cane 32 and the outer hollow cane 33 havebeen removed and further the hollow tubes have been deformed to besubstantially square shaped.

In FIG. 5a is an example of a multi-hollow tube with 7 holes suitablefor use in an embodiment of a preform element of an embodiment of theinvention, such as a preform ring element or a preform center element.

In FIG. 5a is an example of a multi-hollow tube with 7 holes suitablefor use in an embodiment of a preform ring element of an embodiment ofthe invention, such as a preform ring element or a preform centerelement.

In FIG. 5b is an example of a multi-hollow tube with 19 holes suitablefor use in an embodiment of a preform ring element of an embodiment ofthe invention, such as a preform ring element or a preform centerelement.

The multi-hollow tubes are for example produced by stacking a number(corresponding to the desired number of holes) of capillary tubes C andarranging them in a larger hollow tube T and pre-drawing the assemblyunder pressure control of the holes to ensure that they do not collapseand to obtain a desired outer diameter of the multi-hollow tube.Advantageously a very long multi-hollow tube is drawn and thereafter thelong multi-hollow tube is cut into several multi-hollow tubes ofsuitable lengths.

The preform assembly of which the end section is shown in FIG. 6comprises an additional preform center element 41, a preform ringelement 42 of an embodiment of the invention and an additional preformelement in the form of an overcladding tube 43. In an alternative notshown embodiment the preform ring element is switched with an additionalpreform ring element and the additional preform center element is apreform center element according to an embodiment of the invention.

A holder 44 is mounted to hold the overcladding tube 43 as well as theadditional preform center element 41 and the preform ring element 42 inposition. The holder 44 may also be used for mounting a not shownpressure control cap e.g. to a collar section 44 a of the holder.

The preform ring element 42 comprises a slot 45 intersecting a pluralityof holes of the preform ring element 42. The intersected holes areclosed at the first end 42 a of the preform ring element 42.

The first end 41 a of the additional preform center element 41, firstend 42 a of the preform ring element 42 and the first end 43 a of theovercladding tube 43 are sequentially offset at the preform end section,such that an end section of the preform ring element comprising the slot45 protrudes beyond the end of the overcladding tube 43 and such that anend section of the additional preform center element 41 protrudes beyondthe end 42 a of the preform ring element 42.

In a variation of the embodiment shown in FIG. 6 the preform assemblydoes not comprise an overcladding tube and the holder 44 is mounted tohold and control the stability of the preform assembly during drawing,e.g. with a not shown guide tube arranged instead of the overcladdingtube.

The preform assembly of which the end section is shown in FIG. 7acomprises an additional preform center element 51, a preform ringelement 52 of an embodiment of the invention and an overcladding tube53. The end section of the preform assembly is mounted with a pressurecontrolling arrangement comprising a pressure control cap 56 and with apressurization arrangement comprising number of pressure tubesconnections P1, P2, P3 and P4.

The preform ring element 52 comprises a slot 55 intersecting a pluralityof holes 54 of the preform ring element 52. The intersected holes areclosed at the first end 52 a of the preform ring element 52.

The first end 51 a of the additional preform center element 51, firstend 52 a of the preform ring element 52 and the first end 53 a of theovercladding tube 53 are sequentially offset at the preform assembly endsection, such that an end section of the preform ring element comprisingthe slot 55 protrudes beyond the end of the overcladding tube 53 a andsuch that an end section of the additional preform center element 51protrudes beyond the end 52 a of the preform ring element 52.

The overcladding tube 53 is mounted to the pressure control cap 56 atits first end 53 a to thereby provide a number of chambers—here 4chambers C1, C2, C3 and C4 separated by various seals 57.

The additional preform center element 51 comprises a number of not shownholes which terminate at the first end 51 a of the additional preformcenter element 51, such that the not shown holes of the additionalpreform center element 51 are in fluid communication with the a firstchamber C1 such that the pressure within these holes can be controlledin the first chamber C1 via pressure tube connection P1.

The first end 52 a of the preform ring element 52 is located in a secondchamber C2 where the pressure can be controlled via pressure tubeconnection P2 and interface void formed by the preform ringelement-preform center interface is in fluid communication with thesecond chamber C2 to thereby apply a reduced pressure at the preformring element-preform center interface.

The slot 55 is located in a third chamber C3 where the pressure can becontrolled via pressure tube connection P3, such that the intersectedholes of the preform ring element 52 are in fluid communication with thethird chamber C3 via the slot 55, to thereby control the pressure in theintersected holes of the preform ring element 52.

The pressure of the preform ring element-overcladding tube interface isin fluid communication with a fourth chamber C4, such that the pressurecan be controlled via pressure tube connection P4 thereby apply areduced pressure at the preform ring element-overcladding tubeinterface, such that the interface void is closed in the drawn fiber.

Many other configurations may be applied to provide individual controlof respective holes and/or interfaces.

The preform assembly of which the end section is shown in FIG. 7b is avariation of the preform assembly shown in FIG. 7a , where both thepreform center element and the preform ring element is according to anembodiment of the invention. The preform assembly comprises a preformcenter element 58, a preform ring element 52 and an overcladding tube53. The end section of the preform assembly is mounted with a pressurecontrolling arrangement comprising a pressure control cap 56 and with apressurization arrangement comprising number of pressure tubesconnections P1, P2, P3, P4 and P5.

The preform ring element 52 comprises a slot 55 intersecting a pluralityof holes 54 of the preform ring element 52. The intersected holes areclosed at the first end 52 a of the preform ring element 52.

The preform center element 58 comprises a number of the not shownlongitudinal holes which terminate at the first end 58 a of the preformcenter element 58. The preform center element comprises a slot 59intersecting some of the not shown holes of the preform center element58, such as all of the longitudinal holes of the preform center element58 or the radially outermost longitudinal holes of the preform centerelement 58 adapted to provide an outer cladding and/or an air cladding.In the present embodiment the preform center element may have a numberof not intersected longitudinal holes arranged radially inwards to theintersected longitudinal holes. Such not intersected holes may e.g. beadapted to form part of a core region and/or an inner cladding region ofthe final optical fiber drawn from the preform assembly. The intersectedholes are closed at a further slot 59 a which is for example filled witha sealing material such as epoxy or silica or the section above thefurther slot 59 a is simply collapsed.

The first end 58 a of the preform center element 58, first end 52 a ofthe preform ring element 52 and the first end 53 a of the overcladdingtube 53 are sequentially offset at the preform assembly end section,such that an end section of the preform ring element 52 comprising theslot 55 protrudes beyond the end of the overcladding tube 53 a and suchthat an end section of the preform center element 58 comprising the slot59 protrudes beyond the end 52 a of the preform ring element 52.

The overcladding tube 53 is mounted to the pressure control cap 56 atits first end 53 a to thereby provide a number of chambers—here 5chambers C1, C2, C3, C4 and C5 separated by various seals 57.

The not intersected longitudinal holes of the preform center element 58which terminate at the first end 58 a of the preform center element 58are in fluid communication with the a first chamber C1 such that thepressure within these holes can be controlled in the first chamber C1via pressure tube connection P1.

The slot 59 in the preform center element 58 is located in the secondchamber C2 where the pressure can be controlled via pressure tubeconnection P2, such that the intersected holes of the preform element 58are in fluid communication with the second chamber C2 via the slot 59 tothereby control the pressure in the intersected holes of the preformelement 58.

The first end 52 a of the preform ring element 52 is located in thethird chamber C3 where the pressure can be controlled via pressure tubeconnection P3 and the preform ring element-preform center interface isin fluid communication with the third chamber C3 to thereby apply areduced pressure at the preform ring element-preform center interface.

The slot 55 in the preform ring element 52 is located in the fourthchamber C4 where the pressure can be controlled via pressure tubeconnection P4, such that the intersected holes of the preform ringelement 52 are in fluid communication with the fourth chamber C4 via theslot 55, to thereby control the pressure in the intersected holes of thepreform ring element 52.

The pressure of the preform ring element-overcladding tube interface isin fluid communication with the fifth chamber C5, such that the pressurecan be controlled via pressure tube connection P5 thereby apply areduced pressure at the preform ring element-overcladding tubeinterface.

Many other configurations may be applied to provide individual controlof respective holes and/or interfaces.

The assembled preform shown in FIG. 8 is adapted for providing a doubleclad microstructured optical fiber with an inner cladding region and anouter cladding region separated by an air cladding ring. The preformassembly comprises an additional preform center element 61 surrounded bya first preform ring element 62, which is surrounded by a second preformring element 63 which is surrounded by an overcladding tube 64.

The additional preform center element 61 comprises a doped center area61 a for providing the core region of the microstructured optical fiber.

The first preform ring element 62 comprises a number of holes 62 aprovided by hollow tubes or interspace holes as described above. Theholes 62 a are asymmetrically arranged and have the function ofproviding a scrambler-element in the inner cladding region of the finalmicrostructured optical fiber to remove undesired screw rays in theinner cladding region.

The second preform ring element 63 comprises a ring of holes 63 a, whichring of holes in the final microstructured optical fiber provides theair cladding ring which ensures that selected wavelengths of light canbe guided in the inner cladding region.

The holes of the ring of holes 63 a are approximately rectangular with alarge cross dimension and a short cross dimension and arranged withtheir respective large cross dimension substantially in radialdirection. Thereby longer bridges are arranged between adjacent holes ofthe ring of holes, which in the microstructured optical fiber results ina high quality air clad for ensuring guiding light in the inner claddingregion with desired wavelengths e.g. reaching below 2 μm such as below 1μm or even below 500 nm. In an alternatively not shown embodiment theholes of the ring of holes are approximately square or approximatelyrectangular and arranged with their respective shorter cross dimensionsubstantially in radial direction. Thereby the bridge becomes relativelyshort, which may be an advantage if the fiber is to be cleaved.

As it can be seen the respective preform elements, the additionalpreform center element 61, the first preform ring element 62, the secondpreform ring element 63 and the overcladding tube 64 are arranged withsmall gaps G. These gaps G may advantageously be removed before thedrawing of the microstructured optical fiber e.g. by fusion.Alternatively these gaps G will be removed during drawing of themicrostructured optical fiber.

In a variation of the preform assembly of FIG. 8 the additional preformcenter element is according to an embodiment of the invention and one orboth of the preform ring elements is/are additional preform ringelement(s).

The assembled preform shown in FIG. 9 is adapted for providing a doubleclad hollow core microstructured optical fiber with an inner claddingregion and an outer cladding region separated by an air cladding ring.The preform assembly comprises an additional preform center element 71surrounded by a preform ring element 73, which is surrounded by anovercladding tube 74.

The additional preform center element 71 comprises a plurality ofcapillary tubes 71 b arranged to provide a relative large center hole 71a, which in the final microstructured optical fiber will provide thehollow core region.

The preform ring element 73 comprises a ring of holes 73 a, which ringof holes in the final microstructured optical fiber provides the aircladding ring which ensures that selected wavelengths of light can beguided in the inner cladding region.

In a variation of the preform assembly of FIG. 9 the additional preformcenter element is according to an embodiment of the invention and thepreform ring element is an additional preform ring element.

The assembled preform shown in FIG. 10 is adapted for providing a doubleclad solid core microstructured optical fiber with an inner claddingregion and an outer cladding region separated by an air cladding ring.

The preform assembly comprises an additional preform center element 81surrounded by a preform ring element 83, which is surrounded by anovercladding tube 84.

The additional preform center element 81 comprises a plurality ofcapillary tubes 81 b with larger wall thickness than the capillary tubes71 b used in the embodiment of FIG. 9. The capillary tubes 81 b arearranged in a hexagonal pattern to provide that the encircled centerregion 81 a in the final microstructured optical fiber will provide thesolid core region.

The preform ring element 83 comprises a ring of holes 83 a, which ringof holes in the final microstructured optical fiber provides the aircladding ring which ensures that selected wavelengths of light can beguided in the inner cladding region.

In a variation of the preform assembly of FIG. 10 the additional preformcenter element is according to an embodiment of the invention and thepreform ring element is an additional preform ring element.

The assembled preform shown in FIG. 11 is adapted for providing a doubleclad solid doped core microstructured optical fiber with an innercladding region and an outer cladding region separated by an aircladding ring.

The preform comprises an additional preform center element 91 surroundedby a preform ring element 93, which is surrounded by an overcladdingtube 94.

The additional preform center element 91 comprises a doped center region91 a which in the final microstructured optical fiber will provide thesolid doped core region.

The additional preform ring element 93 comprises a ring of holes 93 a,which ring of holes in the final microstructured optical fiber providesthe air cladding ring which ensures that selected wavelengths of lightcan be guided in the inner cladding region.

The microstructured optical fiber drawn from a preform assemblycomprising a preform ring element and as shown in FIG. 12 comprises acore region 101, an inner cladding region 102, an outer cladding region104, and an air clad 103 separating the inner cladding region 103 andthe outer cladding region 104. The air clad 103 originates from a ringof holes in a preform ring element of an embodiment of the invention.The core region is a hollow core and the inner cladding region comprisesa plurality of hole microstructures arranged in a pattern to confinelight to be guided in the hollow core.

The birefringent microstructured optical fiber drawn from a preformassembly comprising a preform ring element and as shown in FIG. 13comprises a core region 111, an inner cladding region 112, an outercladding region 114, and an air clad 113 separating the inner claddingregion 113 and the outer cladding region 114. The air clad 113originates from a ring of holes in a preform ring element of anembodiment of the invention.

The core region is a solid core optionally doped. The inner claddingregion comprises a plurality of hole microstructures 112 a and aplurality of boron doped microstructures 112 b arranged in a twofoldrotational symmetrical pattern to provide the fiber with birefringence.The outer periphery of the microstructured optical fiber has twooppositely planar surface sections 114 a correlated with thebirefringence.

A solid microstructured optical fiber drawn from a preform assemblycomprising a preform ring element and as shown in FIG. 14 comprises acore region 121, an inner cladding region 122, an outer cladding region124, and an air clad 123 separating the inner cladding region 123 andthe outer cladding region 124. The air clad 123 originates from a ringof holes in a preform ring element of an embodiment of the invention.

The core region 121 is a solid doped core. The inner cladding regioncomprises a two relatively large boron doped structures 122 a.

The preform center element shown in FIG. 15 comprises a ring oflongitudinal holes 131 a cladding region and a core region 132. At arelative short distance—e.g. as described above—to a not shown end ofthe preform center element the preform center element comprises anannular slot intersecting holes 131. At a position between the slot andthe first end or at the first end of the preform center element theintersected holes are closed for example using epoxy or another methodas described above. The pressure within the longitudinal holes 131 maythereby be controlled via the slot during drawing of a preform assemblycomprising preform center element to an optical fiber.

The preform center element shown in FIG. 15 comprises a ring oflongitudinal holes 131 a cladding region and a core region 132. At arelative short distance—e.g. as described above—to a not shown end ofthe preform center element the preform center element comprises anannular slot intersecting holes 131. At a position between the slot andthe first end or at the first end of the preform center element theintersected holes are closed for example using epoxy or another methodas described above. The pressure within the longitudinal holes 131 maythereby be controlled via the slot during drawing of a preform assemblycomprising preform center element to an optical fiber. The core region132 may e.g. be a doped core region and/or the cladding region 133 maye.g. be doped and/or comprising microstructures e.g. solidmicrostructure and/or in the form of longitudinal holes which are notintersected by the slot. Such longitudinal holes not intersected by theslot may be pressure controlled from the first end of the preformassembly e.g. as described above e.g. in connection with FIG. 7 b.

During preform assembling the preform center element is assembled withat least an overcladding tube and optionally one or more additionalpreform ring elements between the preform center element and theovercladding tube.

As it can be seen the distance (material thickness t) from the outerpreform element surface 134 to the longitudinal holes 131 is relativelyshort which reduces the risk of damaging the preform center elementduring the cutting of the slot. The required thickness radially outwardsto the longitudinal holes to provide the final fiber with low undesiredleakage of light is thus provided by the overcladding tube andoptionally additional preform ring element(s).

The preform center element shown in FIG. 16 comprises a preform coreregion 141, a preform inner cladding region 142 and a preform outercladding region 143. The preform inner cladding region 142 compriseshexagonally arranged inner cladding longitudinal holes 142 a and thepreform outer cladding region 143 comprises hexagonally arranged outercladding longitudinal holes 143 a.

At a relative short distance—e.g. as described above—to a not shown endof the preform center element the preform center element comprises anannular slot intersecting the outer cladding longitudinal holes 143 a.the dotted circle indicated the depth of the slot extending from thepreform element outer surface 144, such that the inner claddinglongitudinal holes 142 a are not intersected

At a position between the slot and the first end or at the first end ofthe preform center element the intersected holes 143 a are closed forexample using epoxy or another method as described above. The pressurewithin the intersected longitudinal holes 143 a may thereby becontrolled via the slot during drawing of a preform assembly comprisingpreform center element to an optical fiber. The pressure within the notintersected longitudinal holes 142 a may be controlled via their openingat the first end of the preform center element as described above e.g.in connection with FIG. 7 b.

The core region 141 may or may not be doped e.g. in the form of doped ornon-doped silica.

During preform assembling the preform center element is assembled withat least an overcladding tube and optionally one or more additionalpreform ring elements between the preform center element and theovercladding tube.

As it can be seen the distance (material thickness t) from the outerpreform element surface 144 to the intersected longitudinal holes 143 ais relatively short which reduces the risk of damaging the preformcenter element during the cutting of the slot. The required thickness tradially outwards to the longitudinal holes to provide the final fiberwith low undesired leakage of light is thus provided by the overcladdingtube and optionally additional preform ring element(s).

The optical fiber shown in a cross-sectional view in FIG. 17 is drawnfrom a preform assembly comprising a preform center elementcorresponding to the preform center element shown in FIG. 16 but wherethe number off longitudinal holes differs. The optical fiber comprises acore region 151, an inner cladding region 152 and an outer claddingregion 153. The inner cladding region 152 comprises hexagonally arrangedinner cladding longitudinal holes 152 a and the outer cladding region153 comprises hexagonally arranged outer cladding longitudinal holes 153a. A relatively thick silica layer 155 surrounds the outer claddingregion 153. The image only show a part of the silica layer 155. Theactually thickness of the silica layer 155 is about 5 times thicker thanshown and provides the optical with a round outer periphery. The majorpart of the silica layer 155 originates from an overcladding tube whichis fully fused to the preform center element during the drawing of thefiber.

The optical fiber shown in a cross-sectional view in FIG. 18 is drawnfrom a preform assembly comprising a preform center element according toan embodiment of the invention. The optical fiber comprises 4 coreregions 161 surrounded by 4 partially overlapping cladding regions 162and an air cladding 163. The 4 partially overlapping cladding regions162 comprises cladding region holes 162 a arranges in hexagonal patternsaround each of the 4 core region 161. The air cladding comprises adouble layer of air cladding holes arranged in a ring with an octagonalshape. The octagonal shape of the air cladding 163 is provided to ensuresingle mode transmission in the optical fiber while at the same timehaving a few as possible holes in the cladding region 162. Further ithas been found that the octagonal shape of the air cladding 163 may makethe stacking of longitudinal structures of the preform center elementsimpler. A relatively thick silica layer 165 surrounds the air cladding163. The image only show a part of the silica layer 165. The actuallythickness of the silica layer 165 is about 2-5 times thicker than shownand provides the optical with a round outer periphery. The major part ofthe silica layer 155 originates from an overcladding tube which is fullyfused to the preform center element during the drawing of the fiber. Thecore regions 161, the cladding regions 162 and the air claddingoriginate from the preform center element where the slot of the preformcenter element intersects the hollow tubes forming the air cladding 163.

Embodiments

1. A preform element for a fiber preform, said preform element has alength and a center axis along its length, a first and second enddefined by its length and an outer preform element surface and saidpreform element comprising a plurality of longitudinal structuresdisposed to form longitudinal hole(s) in a background material, saidpreform element comprises at least one slot extending from its outerpreform element surface and intersecting at least one of saidlongitudinal holes and wherein the at least one slot does not fullyintersect the preform element, preferably said plurality of longitudinalstructures are encaged in and/or forms said background material andextending in the length direction of the preform element, wherein one ormore of said plurality of longitudinal structures form(s) saidlongitudinal hole(s), preferably at least a part of the backgroundmaterial is provided by solid longitudinal structures and/or by materialof hollow longitudinal structures.

2. The preform element of embodiment 1, wherein one or more of saidplurality of longitudinal structures are hollow tubes with saidlongitudinal holes and/or said longitudinal holes are longitudinalinterspace holes formed between said longitudinal structures, whereinsaid least one slot extending from its outer preform element surfaceintersects at least one of said holes of said hollow tubes and/or atleast one of said interspace holes.

3. The preform element of embodiment 1 or embodiment 2, wherein saidbackground material is glass, such as silica glass, soft glasschalcogenide glass or any combinations thereof, optionally thebackground material comprises one or more dopants, such as one or more,index-changing dopants, such as Ge, B, F, P, Al and/or one or moreactive material dopants, such as the rare-earth elements Er or Yb.

4. The preform element of any one of the preceding embodiments, whereinthe longitudinal structures are arranged substantially parallel to thecenter axis of the preform element, preferably said preform elementcomprises at least one ring of hollow tubes.

5. The preform element of any one of the preceding embodiments, whereinthe hollow tubes each comprise a plurality of longitudinal holessubstantially parallel to the center axis of the preform element, saidplurality of holes of each hollow tube are preferably arranged in apredetermined cross sectional pattern comprising at least one ring ofholes and optionally a center hole.

6. The preform element of any one of the preceding embodiments, whereinthe hollow tubes are of glass, such as silica glass, soft glasschalcogenide glass or any combinations thereof, optionally the hollowtube glass comprises one or more dopants, such as one or more,index-changing dopants, such as Ge, B, F, P, Al and/or one or moreactive material dopants, such as the rare-earth elements Er or Yb. 7.The preform element of any one of the preceding embodiments, whereinsaid preform element has n fold rotational symmetry with respect torotations about its center axis, wherein n is up to 500, such as up to100, such as up to 50, such as from 2 to 20.

8. The preform element of any one of the preceding embodiments, whereinsaid longitudinal structures extend in substantially the entire lengthof the preform element.

9. The preform element of any one of the preceding embodiments, whereinsaid at least one slot is in the form of a radially inwards cut in thepreform element from its outer preform element surface, wherein theradially inwards cut is substantially perpendicular to the center axis.

10. The preform element of any one of the preceding embodiments, whereinsaid at least one slot is positioned at a distance from the first end ofthe preform element which is preferably up to 1/10 of the length of thepreform element, said distance is preferably at least about 0.5 mm orlonger, at least about 5 mm or longer, such as about 1 cm to about 20cm.

11. The preform element of any one of the preceding embodiments, whereinsaid intersected holes are closed at a position from the slot to thefirst end, such as at said first end of the preform and/or at theintersection towards the first end and/or a position there between.

12. The preform element of any one of the preceding embodiments, whereinsaid preform element is adapted to form part of a preform for amicrostructured optical fiber, wherein said preform element is adaptedfor being assembled with at least one additional preform element.

13. The preform element of any one of the preceding embodiments, whereinthe distance from said outer preform element surface to an radiallyoutermost of said at least one intersected hole is less than about ⅛ ofthe preform element diameter, such as less than about 1/10 of thepreform diameter, such as less than 1 cm, such as less than about 0.5cm, such as less than about 3 mm.

14. The preform element of any one of the preceding embodiments, whereinthe preform element is a preform center element comprising at least apreform core region and a preform cladding region surrounding thepreform core region, wherein said plurality of longitudinal structuresforming said longitudinal holes are disposed in said preform claddingregion, said holes are preferably arranged in a cross-sectionalhexagonal or octagonal pattern.

15. The preform element of embodiment 14, wherein the preform claddingregion comprises an inner preform cladding region surrounding thepreform core region and an outer preform cladding region surrounding theinner preform cladding region, wherein said plurality of longitudinalstructures forming said longitudinal holes are disposed in said outerpreform cladding region.

16. The preform element of embodiment 14 or embodiment 15, wherein theplurality of longitudinal structures are disposed to form a ring oflongitudinal holes in said preform cladding region, preferably said ringof longitudinal holes are adapted for providing an air cladding in afiber drawn from a preform comprising said preform element.

17. The preform element of any one of the preceding embodiments, whereinthe at least one slot is an annular slot intersecting a plurality ofsaid holes of said hollow tubes and/or all of said interspace holes,preferably, such as substantially all of said holes of said hollow tubesand/or all of said interspace holes of the preform cladding region orthe outer preform cladding region are intersected by said annular slot.

18. The preform element of any one of embodiments 1-13, wherein saidpreform element is a preform ring element comprising wherein saidpreform ring element has an inner preform element surface and said outerpreform surface is in the form of an outer preform ring element surface,said background material is in the form of a ring wall backgroundmaterial, said slot intersecting said at least one of said longitudinalholes extending from its outer preform element surface and wherein theat least one slot does not fully intersect the ring wall.

19. The preform element of embodiment 18, wherein said ring wallbackground material comprises a ring wall inner background material anda ring wall outer background material, which inner and outer backgroundmaterial may be equal or may different from each other.

20. The preform element of any one embodiments 18 and 19, wherein saidpreform ring element comprises a ring of said longitudinal structuresarranged with interspace between adjacent longitudinal structures toform said one or more longitudinal interspace holes, said longitudinalstructures encaged in the ring wall background material optionallycomprises one or more solid structures, such as at least one solidstructure having a refractive index different from an index of the ringwall background material.

21. The preform element of any one of embodiments 18-20, wherein saidpreform ring element comprises a ring of hollow tubes, preferably thehollow tubes each comprise a plurality of longitudinal holessubstantially parallel to the center axis of the preform ring element,said plurality of holes of each hollow tube are preferably arranged in apredetermined cross sectional pattern comprising at least one ring ofholes and optionally a center hole.

22. The preform element of any one embodiments 18-21, wherein said ringwall has a substantially identical thickness along its annular extensionat least in a cross section, preferably in its whole length, morepreferably said preform ring element is substantially circularsymmetrical around its center axis with respect to thickness.

23. The preform element of any one of the embodiments 18-22, whereinsaid ring wall has an average inner diameter (ID) defined by the innerpreform element surface and an average outer diameter (OD) defined bysaid outer preform element surface, said ID/OD in at least a crosssection of the preform ring element is from about 0.3 to about 0.95,such as from about 0.7 to about 0.9. 24. The preform element ofembodiment 23, wherein said ID and said OD are substantially constantalong the length of the preform ring element, preferably the thicknessof the preform ring element defined by 0.5 times OD minus ID is fromabout 0.3 mm to about 2 cm, such as from about 1 mm to about 1 cm, suchas from about 3 mm to about 6 mm.

25. The preform element of any one of the embodiments 18-24, whereinsaid ring wall comprises an inner hollow cane, an outer hollow cane andsaid plurality of longitudinal structures encaged between said innercane and said outer cane.

26. The preform element of embodiment 25, wherein the inner hollow canehas an inner cane wall thickness and said outer cane has an outer canewall thickness, wherein said inner cane wall thickness and said outercane wall thickness independently of each other are from about 1 mm toabout 1 cm, such as from about 2 mm to about 5 mm.

27. The preform element of any one of the embodiments 18-26, whereinsaid plurality of longitudinal structures are arranged in a side by sideconfiguration along the ring shape of the ring shaped wall to form atleast one ring of longitudinal structures, said ring of longitudinalstructures comprises a plurality of said longitudinal holes intersectedby said at least one slot, optionally said plurality of longitudinalstructures are arranged to form at least two rings of longitudinalstructures, such as three rings of longitudinal structures.

28. The preform element of any one of the embodiments 18-27, wherein theat least one slot is an annular slot intersecting a plurality, such assubstantially all of said holes of said hollow tubes and/or all of saidinterspace holes.

29. The preform element of any one of the embodiments 18-28, whereinsaid preform ring element has an inner diameter of at least about 3 mm,such as at least about 5 mm, such as at least about 10 mm, such as fromabout 15 to about 50 mm.

30. The preform element of any one of the preceding embodiments, whereinsaid preform element is obtainable by a method of any one of embodiments31-51

31. A method of producing a preform element of any one of the precedingembodiments, wherein said method comprises providing a plurality oflongitudinal structures and disposing said longitudinal structures toform longitudinal hole(s) in a background material said method comprisesproviding at least one slot extending from said outer preform elementsurface and intersecting at least one of said longitudinal holes andwherein the at least one slot does not fully intersect the preformelement.

32. The method of embodiment 31, wherein the method comprises, whereinsaid method comprises arranging said longitudinal structures to providesaid plurality of longitudinal holes substantially parallel to thecenter axis of the preform element.

33. The method of embodiment 31 or embodiment 32, wherein the methodcomprising providing said at least one slot is in the form of a radiallyinwards cut in the preform element from its outer preform elementsurface, wherein the radially inwards cut is substantially perpendicularto the center axis.

34. The method of any one of embodiments 31-33, wherein said at leastone slot is provided at a distance from the first end of the preformelement which is preferably up to 1/10 of the length of the preformelement, said distance is preferably at least about 0.5 mm or longer, atleast about 5 mm or longer, such as about 1 cm to about 20 cm.

35 The method of any one of embodiments 31-34, wherein the methodfurther comprises closing said intersected holes at a position from thefirst end of the preform to the intersection, such as at the first endof the preform and/or at the intersection towards the first end of thepreform and/or at an additional slot intersecting the holes, saidintersected holes are preferably closed by collapsing and/or by fillinga solid material into the holes.

36. The method of any one of embodiments 31-35, wherein said methodcomprises stacking said longitudinal structures and optionally arrangingthe stacked longitudinal structures in an outer cane. 37. The method ofany one of embodiments 31-36, wherein said preform element is a preformcenter element and the method comprises stacking said longitudinalstructures to form at least said preform core region and said preformcladding region surrounding the preform core region, wherein saidplurality of longitudinal structures forming said longitudinal holes aredisposed in said preform cladding region, preferably the methodcomprises arranging said holes in a cross-sectional hexagonal oroctagonal pattern.

38. The method of embodiment 37, wherein the method comprises arrangingsaid longitudinal structures to form said preform cladding region tocomprise an inner preform cladding region surrounding the preform coreregion and an outer preform cladding region surrounding the innerpreform cladding region.

39. The method of embodiment 37 or embodiment 38, wherein the methodcomprises disposing said plurality of longitudinal structures to form aring of longitudinal holes in said preform cladding region, preferablysaid ring of longitudinal holes are adapted for providing an aircladding in a fiber drawn from a preform comprising said preformelement.

40. The method of any one of embodiments 31-39, wherein the methodcomprises performing said at least one slot is an annular slotintersecting a plurality of said holes of said hollow tubes and/or allof said interspace holes, preferably, such as half of or substantiallyall of said holes of said hollow tubes and/or all of said interspaceholes of the preform cladding region or the outer preform claddingregion are intersected by said annular slot.

41. The method of any one of embodiments 31-36, wherein said preformelement is a preform ring element, said method comprises providing anouter ring wall section, an inner ring wall section and a plurality oflongitudinal structures and encaging said longitudinal structures inbetween said outer ring wall section and said inner ring wall section toform longitudinal holes and providing said at least one slot extendingthrough the outer ring wall section and intersecting at least one ofsaid longitudinal holes. 42. The method of embodiment 41, wherein saidouter ring wall section is an outer hollow cane and said inner ring wallsection is an inner hollow cane, preferably said outer hollow cane hasan outer surface forming an outer preform element surface and saidmethod comprises performing said at least one slot extending from theouter preform element surface and intersecting said longitudinalhole(s).

43. The method of embodiment 41 or embodiment 42, wherein said encagingof said at least one longitudinal structure in between said outer ringwall section and said inner ring wall section comprises assembling saidouter ring wall section and said inner ring wall section with saidlongitudinal structures in between and at least partly fusing said outerring wall section and said inner ring wall section together via saidlongitudinal structures.

44. The method of any one of embodiments 41-42, wherein said assemblingcomprises arranging said inner wall section with its outer surface ofsaid inner ring wall to face an inner surface of said outer ring walland arranging said longitudinal structures between said outer surface ofsaid inner ring wall and said inner surface of said outer ring wall.

45. The method of any one of embodiments 41-44, wherein the inner andouter ring wall sections are hollow canes and the assembling comprisesinserting said inner ring wall section into said outer ring wall sectionand arranging a plurality of said longitudinal structures between saidinner ring wall section and said outer ring wall section.

46. The method of any one of embodiments 41-45, wherein the methodcomprises arranging said longitudinal structures in a side by sideconfiguration to surround said inner ring wall section, to form at leastone ring of longitudinal structures, such as two rings of longitudinalstructures, such as three rings of longitudinal structures.

47. The method of any one of embodiments 41-46, wherein one or more ofsaid plurality of longitudinal structures are hollow tubes with saidlongitudinal holes and/or said longitudinal holes are longitudinalinterspace holes between said longitudinal structures, wherein saidleast one slot is provided to intersects at least one of said holes ofsaid hollow tubes and/or at least one of said interspace holes,preferably all of said longitudinal structures are hollow tubesoptionally including one or more tubes with a plurality of longitudinalholes substantially parallel to the center axis of the preform ringelement.

48. The method of embodiment 46 or embodiment 47, wherein the ring oflongitudinal structures is arranged in a configuration where saidrespective longitudinal structures are in contact with adjacentlongitudinal structures along their lengths.

49. The method of embodiment 46 or embodiment 47, wherein the ring oflongitudinal structures is arranged in a configuration where saidrespective longitudinal structures are not in contact with adjacentlongitudinal structures thereby forming said longitudinal holes in theform of interspace longitudinal holes, preferably the method comprisesarranging a distance arrangement at two or more positions along thelength of the preform ring element for controlling the distance betweenthe longitudinal structures.

50. The method of any one of embodiments 41-49, wherein said at leastone slot is provided by a radially inwards cut through the outer ringwall section, wherein the radially inwards cut is substantiallyperpendicular to the center axis.

51. The method of any one of embodiments 41-49, wherein the methodfurther comprises closing said intersected holes at a position from thefirst end of the preform to the intersection, such as at the first endof the preform and/or at the intersection towards the first end of thepreform and/or at an additional slot intersecting the holes, saidintersected holes are preferably closed by collapsing and/or by fillinga solid material into the holes.

52. A method of producing a microstructured optical fiber, the methodcomprising assembling a preform, and drawing the preform assembly to amicrostructured optical fiber, wherein the assembling of the preformcomprises providing a preform element according to any one of theembodiments 1-22 and assembling it with at least one additional preformelement.

53. The fiber production method of embodiment 52, wherein the at leastone additional preform element is an overcladding tube with a length anda first and a second end and a bore and the preform assembling comprisesdisposing said preform element into said bore of said overcladding tubeto form a preform element-overcladding tube interface, wherein thepreform assembly has a first end section comprising said first end ofsaid preform element and said first end of said overcladding tube.

54. The fiber production method of embodiment 52 or embodiment 53,wherein said preform element is a preform center element which isassembled with an additional preform ring element and/or an overcladdingtube or wherein said preform element is a preform ring element which isassembled with an additional preform center element and optionally anovercladding tube.

55. The fiber production method of embodiment 54 wherein the methodcomprises providing a preform center element with a length and a firstand a second end, and arranging the ring shaped wall to surround saidpreform center element to form a preform ring element-preform centerinterface, wherein the preform assembly has a first end sectioncomprising said first end of said preform center element and said firstend of said preform ring element.

56. The fiber production method of embodiment 55, wherein said preformcenter element is a preform center element according to any one ofembodiments 1-17.

57. The fiber production method of embodiment 56, wherein the assemblingof the preform comprises providing an additional preform element in theform of an overcladding tube with a length and a first and a second endand a bore and arranging said ring shaped wall into said bore of saidovercladding tube to form a preform ring element-overcladding tubeinterface, wherein the preform assembly has a first end sectioncomprising said first end of said preform center element, said first endof said preform ring element and said first end of said overcladdingtube.

58. The fiber production method of any one of embodiments 52-57, whereinat least one of said elements interface(s) is subjected to a reducedpressure during at least a part of the drawing of the preform assemblyto a microstructured optical fiber, preferably at least one of saidpreform element-overcladding tube interface and said preform ringelement-preform center interface is/are subjected to a reduced pressureduring at least a part of the drawing of the preform assembly to amicrostructured optical fiber, wherein said preform element-overcladdingtube interface is selected from a preform center element-overcladdingtube interface and a preform ring element-overcladding tube interface59. The fiber production method of any one of embodiments 52-58, whereinthe preform center element comprises at least one hole extending alongits length, said method comprising subjecting said at least one hole ofsaid preform center element to a preform center controlled pressureduring the drawing, preferably the preform center element is accordingto any one of embodiments 1-17 and the controlled pressure is appliedvia the slot of the preform center element.

60. The fiber production method of any one of embodiments 52-59, whereinthe preform center element comprises at least a first category hole andat least a second category hole extending in length of the preformcenter element, said method comprising subjecting said at least onefirst category hole to a first preform center controlled pressure andsubjecting said at least one second category hole to a second preformcenter controlled pressure during the drawing.

61. The fiber production method of any one of embodiments 55-60, whereinthe method comprises subjecting said longitudinal holes of said preformring element to a preform ring element controlled pressure during thedrawing, wherein said preform ring element controlled pressure iscontrolled independently of at least one other controlled pressureduring the drawing, such as said preform center controlled pressure. 62.The fiber production method of any one of embodiments 55-61, wherein themethod comprises subjecting said longitudinal holes of said preform ringelement intersected by said slot to a preform ring element controlledpressure during the drawing, wherein said preform ring elementcontrolled pressure is applied via said slot.

63. The fiber production method of any one of embodiments 52-62, whereinsaid first end of said preform center element and said first end of saidovercladding tube are sequentially offset at said preform end section,such that said end section of said preform center element protrudesbeyond the end of said overcladding tube.

64. The fiber production method of any one of embodiments 54-63, whereinsaid first end of said additional preform center element, said first endof said preform ring element and said first end of said overcladdingtube are sequentially offset at said preform end section, such that saidend section of said preform ring element protrudes beyond the end ofsaid overcladding tube and preferably such that an end section of saidadditional preform center element protrudes beyond the end of saidpreform ring element.

65. The fiber production method of any one of embodiments 55-64, whereinsaid first end of said preform center element, said first end of saidadditional preform ring element and said first end of said overcladdingtube are sequentially offset at said preform end section, such that saidend section of said additional preform ring element protrudes beyond theend of said overcladding tube and preferably such that an end section ofsaid preform center element protrudes beyond the end of said additionalpreform ring element.

66. The fiber production method of any one of embodiments 51-65, whereinsaid one or more pressure controls are provided by enclosing said endsection of said preform assembly by a pressure control cap comprisingtwo or more pressure control sections and applying seals to seal offindependently pressure controlled holes, comprising at least a pair ofslot seals on opposite sides of said slot of said preform element,preferably said intersected holes of said preform element are closed ata position from said first end of the preform to said slot intersectiontowards the first end and said pair of slot seals forms annular sealsagainst the outer preform element surface of said end section of saidpreform element.

67. The fiber production method of any one of embodiments 51-66, whereinthe drawing of said preform assembly comprises drawing of the preformassembly to said microstructured optical fiber in a drawing tower,wherein the preform assembly is subjected to at least one pressurecontrol at its first end section, preferably two or more independentlycontrolled pressure controls.

68. A method of forming an optical fiber comprising:

-   -   forming an optical fiber preform assembly comprising at least        one interface void and comprising a plurality of longitudinal        holes and one or more longitudinally extending voids; and    -   drawing the optical fiber preform assembly into the optical        fiber while pressuring at least one of the holes through a        radially inwardly extending slot in the preform assembly and        while evacuating said at least one interface void.

69. The fiber production method of embodiment 68, wherein the method isaccording to any one of the embodiments 51-67.

70. The fiber production method of embodiment 68 or embodiment 69,wherein, the method comprises drawing the optical fiber, such that theat least one interface void is closed in the drawn fiber.

1. A preform assembly for a microstructured optical fiber, the fiberpreform assembly comprising: a preform center element, and a preformring element arranged to surround the preform center element, where saidpreform ring element has a length and a center axis along its length, afirst and second end defined by its length and an outer preform elementsurface and said preform ring element comprising a plurality oflongitudinal structures disposed to form longitudinal hole(s) in abackground material, said preform ring element comprises at least oneslot extending from its outer preform element surface and intersectingat least one of said longitudinal holes and wherein the at least oneslot does not fully intersect the preform ring element, wherein saidplurality of longitudinal structures is encaged in said backgroundmaterial and extends in the length direction of the preform element, andwherein the preform ring element is provided by arranging an innerhollow cane inside an outer hollow cane and arranging the longitudinalstructures between the inner and outer cane.
 2. The preform assembly ofclaim 1, wherein one or more of said plurality of longitudinalstructures are hollow tubes with said longitudinal holes and/or saidlongitudinal holes are longitudinal interspace holes formed between saidlongitudinal structures, wherein said least one slot extending from itsouter preform element surface intersects at least one of said holes ofsaid hollow tubes and/or at least one of said interspace holes.
 3. Thepreform assembly of claim 1, wherein the longitudinal structures arearranged substantially parallel to the center axis of the preformelement.
 4. The preform assembly of claim 3, wherein said preform ringelement comprises at least one ring of hollow tubes.
 5. The preformassembly of claim 1, wherein said at least one slot is positioned at adistance from the first end of the preform element.
 6. The preformassembly of claim 5, wherein said distance is at least about 5 mm. 7.The preform assembly of claim 1, wherein said at least one slot ispositioned at a distance from the first end of the preform element whichis up to 1/10 of the length of the preform element.
 8. The preformassembly of claim 1, wherein the plurality of longitudinal structuresare disposed to form a ring of longitudinal holes in a preform claddingregion.
 9. The preform assembly of claim 8, wherein said ring oflongitudinal holes are adapted for providing an air cladding in a fiberdrawn from said preform assembly.
 10. The preform assembly of claim 1,wherein the at least one slot is an annular slot intersecting aplurality of said holes of said hollow tubes and/or a plurality of saidinterspace holes are intersected by said annular slot.
 11. The preformassembly of claim 1, wherein said intersected holes are closed at aposition from the slot to the first end, such as at said first end ofthe preform and/or at the intersection towards the first end and/or aposition there between.
 12. The preform assembly of claim 1, wherein thepreform center element comprises a plurality of center longitudinalholes that are not intersected by the slot, the plurality of centerlongitudinal holes terminating at a first end of the preform centerelement.
 13. The preform assembly of claim 1, wherein at least a part ofthe background material is provided by solid longitudinal structuresand/or by material of hollow longitudinal structures.
 14. A method ofproducing a microstructured optical fiber, the method comprisingassembling a Preform, and drawing the preform assembly to amicrostructured optical fiber, wherein the assembling of the preformcomprises providing a preform assembly according to claim
 1. 15. Themethod of claim 14, wherein the method comprises subjecting saidlongitudinal holes of said preform ring element intersected by the slotto a preform ring element controlled pressure during the drawing wheresaid preform ring element controlled pressure is applied via said slot,wherein said preform ring element controlled pressure is controlledindependently of at least one other controlled pressure during thedrawing, such as a preform center controlled pressure subjected tolongitudinal holes in the preform center element.
 16. The method ofclaim 14, wherein said one or more pressure controls are provided byenclosing said end section of said preform assembly by a pressurecontrol cap comprising two or more pressure control sections andapplying seals to seal off independently pressure controlled holes,comprising at least a pair of slot seals on opposite sides of said slotof said preform element.
 17. The method of claim 14, wherein said one ormore pressure controls are provided by enclosing said end section ofsaid preform assembly by a pressure control cap comprising two or morepressure control sections and applying seals to seal off independentlypressure controlled holes, comprising at least a pair of slot seals onopposite sides of said slot of said preform element, wherein saidintersected holes of said preform element are closed at a position fromsaid first end of the preform to said slot intersection towards thefirst end and said pair of slot seals forms annular seals against theouter preform element surface of said end section of said preformelement.